Slay the Spire Downfall – The unofficial expansion mod

Video Version of this Article

Slay the Spire: Downfall is a huge mod that adds an entirely new game mode where you switch sides to defend the heart from the vanilla characters trying to climb the spire. You do this as one of the four new classes you can now play in this mode, being all three of the act 1 bosses starting with the slime boss, and even as a snecko! This mod is meant to be like a complete expansion to slay the spire, keeping everything world-friendly, and adding a ton of new content.

You begin at the very top of the act, as the boss normally would, it’s your job to descend the floors of the spire to reach the bottom where one of the four vanilla characters will be waiting to fight you.
The hallway and elite fights along the way will all remain the same, the main difference in this downfall mods comes with the new classes, boss fights, and many new events, all of this content being incredibly well designed and fun to play.


Like I said, the slime boss is the very first class you’ll have unlocked with this mod before you beat act 1 to unlock the next class. This class is actually an updated and heavily reworked version of the slimebound mod by modder michael mayhem, most of the classes of downfall are heavily reworked and re-balanced versions of his old mods, which is really awesome to see all of his work come together in one package. The classic slimebound class mod is still available for those who still want to play it, this slime boss class keeps a lot of the same ideas and archetypes, but changes up a whole bunch of things.

The heart of goo starting relic now makes you heal 2 hp whenever you consume any goop. Your starting deck comes with corrosive spit which applies 6 goop for 1 energy, a stacking debuff that simply makes your next attack deal that much extra damage.

There are a ton of cards that apply and manipulate goop, like a whole set of different licks that apply a small amount of goop alongside other effects like gaining block, applying vulnerable, and much more for 0 energy each. There are a lot of different attacks that gain a bonus effect when consuming goop, like leeching strike gaining block equal to the amount of goop consumed.

Splitting into different slimes is a huge mechanic for this class, you start off with the basic split card, that let’s you choose one of the four basic slimes to split into. The options include a bruiser slime who attacks for 5 each turn, guerrilla slime who attacks all enemies for 3 each turn, the mire slime who attacks for 2 and applies 2 goop each turn, or the leeching slime who attacks for 2 and gives you 2 block per turn. This card can be upgraded to split into 2 slimes instead of 1, you only have three slots to split slimes into, and if you spawn more beyond that you’ll absorb the oldest slime and gain strength. These slimes are an amazingly powerful tool for this class once you start getting some other cards to synergize with them, most importantly any card with the keyword command will cause your front slime to attack that many times, and cards that give you potency will increase the damage of these slimes. There’s a common card for each of these basic slimes that summon one and command up to 3 times, but these aren’t the only slimes available, there’s quite a few special ones from events or colorless cards, like the greed ooze who will steal your money when you visit a campsite in exchange for damage, the darkling slimes who will spawn and attack as a group of 3, and a few more!
The rest of your deck is the basic 4 defends and 3 strikes. There’s only three strikes in the starter deck because one of them is replaced with tackle, a 1 energy attack that deals 10 damage, and three damage to yourself. This tackle seems really simple at first glance, but it’s really a whole other archetype you can build around, with a bunch more tackles that deal heavy damage to the enemy and to yourself, cards like roll through which negated the self-inflicted damage of the next few tackles, and powers like recklessness increasing the damage of all tackles by 3.
You’ll often be using your HP as a resource while playing the slime boss, because whenever you lose max HP from splitting, you lose HP for the fight, being healed back at the end.


The four new classes are all pretty awesome, but one of the biggest features of this mod are the new boss fights against the original 4 characters, and I’d hate to go so long in this video without talking about it in detail, so I’ll stagger all these features and classes in this video, to keep things interesting.
Since you’re playing as an act boss yourself, the bosses that you’ll be facing are the original four playable characters, the ironclad, the silent, the defect, and the watcher, all chosen randomly for each act. These bosses have their own set of relics and cards just as you would if you were playing vanilla. Their starting relic, Neow’s Blessing, slightly changes the basic mechanics of the game, instead of gaining three energy and drawing 5 cards a turn, these bosses will gain 2 energy and 3 cards a turn instead. This relic also gives them 100 max hp per act climbed to make sure they are balanced for the act they show up in. They’ll also have a bunch of other relics, even including ones showing you which events they took. You can see every card they draw just above them, the cards they plan on playing will be highlighted and shown the normal intent whether it be damage, block, both, or a power. The rest of their hand will be greyed out and discarded at the end of their turn, like normal.

The first act bosses are at a point similar to where you’d be at the end of act 1, just taking whatever cards you need to defeat the boss, no real archetype, but still pretty dangerous with cards like perfect strike, .
In act two you’ll start facing these bosses with more relics, and a set powerful build. For example you’ll see things like a claw defect with dolly’s mirror copying claw, a shiv spamming silent with kunai, and a strength scaling ironclad with a very hard hitting heavy strike, or a retain watcher that’ll buff up her cards as she keeps them in her hand.
As your run progresses through act 3, the bosses decks become extra powerful, their decks and collection of relics are something that you would kill for if you were slaying the spire. You’ll face off against a block ironclad with calipers, juggernaut and 60 damage body slams, a poison silent who will apply a crazy amount of poison with bouncing flask and catalyst, or a defect that will gain a ton of orb slots with inserter then spam orbs with barrage, or a watcher that will very quickly stack up mantra for divinity stance to wreck you in one turn. These act 3 boss fights are really tough, maybe even a little unfair especially when you consider that poison will go through your block at the beginning of your turn, but this unfairness can be circumvented by collecting the three keys, and breaking them at the campfire to get these new relics. These relics give you basic stats like max hp, strength, and dexterity, but they also give you a copy of this special retain card in the act 3 boss fight for each relic. Against the silent these cards will make you lose all poison, and make poison no longer go through block, against the defect it will remove all of the enemy’s orbs, against the ironclad it will remove all of the enemy’s block, and against the watcher it will force her out of her stance and make her lose all mantra.

Every single one of these bosses has some form of scaling, as you’d need in vanilla, and will block a lot more often than the vanilla classes, especially when they have relics like orialchium or calipers.
There is so much variety in these bosses since all four characters have different archetypes they’ll build around.
I’ve seen other mods and even other games try this concept of fighting an enemy with their own deck of cards and hand to draw in, but I’ve never seen it done as smoothly and easy to understand as it’s been done in this mod. These boss fights are really fun to play against.
One thing that bums me out is that every single boss is a single-target fight, so taking AOE feels bad sometimes.


The Guardian Class is unlocked after beating act 1 with the slime boss. Just like you’d expect, shifting modes, blocking, and gaining a ton of thorns is a big part of his kit, but he also has some other new and super interesting mechanics like the gem socketing and the stasis system.
Let’s begin with his starting relic, mode shifter, when the guardian takes 10 damage he’ll shift into defensive mode for the next turn. Defensive mode is a buff that lasts 1 turn, very commonly used by this class that gives you three thorns and 2 block for every card played while in that mode. There are a bunch of ways to enter defensive mode, that mode shifter starting relic, the 0 energy curl up starting card, and so many different cards such as piercing hide giving block, thorns, and entering defensive mode. There are a bunch of ways to further buff defensive mode through several powers like Evasive protocal giving extra dexterity, spiker protocol that gives extra thorns, and even with a bunch of relics like baalor’s lordly plate giving you 1 additional block per card played. The other starting card in the guardian’s deck is twin slam, an attack dealing 4 damage twice and has an open socket, you can upgrade it to have an extra socket. You may be wondering what these sockets are, this is probably one of the coolest mechanics in this mod in my opinion. Throughout all of your card rewards and shops, you’ll have a chance of encountering a bunch of different gems, 0 mana cards that do a super simple thing like gaining 1 momentary strength, entering defensive mode, gaining 1 energy, and much more. These gems aren’t that great by themselves, sure they cost 0 energy, but they cost you the card draw which is a really important resource. The true power of these gems come in socketing, every rest site you visit allows you to enhance as a free action, socketing any number of gems you have in your deck into cards with empty sockets. Your starting deck has the twin strike as mentioned with 2 sockets, and the curl card can be upgraded to hold 1 socket, but these two cards are actually pretty terrible and not really worth socketing gems into, but you’ll come across a few amazing cards like prismatic barrier and prismatic beam dealing 5 damage or gaining 4 block per gem socketed up to 3 times, bauble beam that deals some damage and can procc two different gems 3 times, and things like crystal beam which will deal extra damage for each socketed gem in your deck. The other big new mechanic with this guardian class is Stasis, which is what these three slots are for. You don’t start with any stasis capabilities, but you’ll come across a lot of common cards that allow you to utilize this, like recover giving block and placing a card from your discard into stasis, planning placing the top 2 cards of your draw pile into stasis, and reroute dealing 9 damage and putting the next played card into stasis. All of your cards in stasis will start with a counter equal to it’s energy cost plus 1, this counter will tick down every turn until it hits 0, then the card will go into your hand and cost 0 for that turn. This is a decently powerful mechanic by itself, nobody’s going to complain about getting extra 0 cost cards even if you have to wait a while, but stasis can become much stronger when you pair it up with a bunch of synergistic cards like temporal strike and shield having extra effects if you have a card in stasis, emergency allowing you to accelerate the next stasis card into your hand, and some relics like cryo chamber giving you 1 stasis slot and upgrading every card that enters stasis. There’s a few cards that get stronger as they are in stasis using the TICK keyword, like shield charger giving you block and energy every time it ticks down, and multi-bean increasing the damage dealt by any beam card by 1 with every tick in stasis.
Another huge archetype for the guardian is strength scaling, which is a lot easier to understand than gems and stasis since it’s a vanilla mechanic. A card that I’ve had a lot of success with is spot weakpoint, putting a debuff on the enemy that makes you gain 1 temporary strength every time you hit them. Pairing this up with powers like floating orbs giving you a free 2 hit card every turn and attacks walker claw getting affected by strength two times has absolutely destroyed for me.
I really like the guardian class, I feel like it went way above and beyond what you’d expect from a normal class mod with these super interesting and unique mechanics, you can have a lot of success with him building many different ways such as full defense, full strength, a million gems, and more.


Now let’s talk about the events in this mod. I think the events in downfall are absolutely the best events I’ve ever experienced in the several act mods that I’ve played so far. There are a ton of events specific to the class you’re playing like the one that gives you darkling slimes as the slime boss, and one that gives you a pickaxe and allows you to mine for gems as the guardian, spending your HP if you run out of pickaxe charges. These brand new events are allright, most of them add some really cool new mechanics through relics or cards, but what really shines to me are these events that put a new spin on the vanilla events that we’re so used to.
Most of these guys that approach you like the clerics. Gremlins, and augmenters will re-act differently to you, since you are now a big giant boss monster.

If you don’t like what you landed on in the wheel event, you can yell at the gremlin until he calls his nob bodyguard to fight you, if you win you’ll receive the wheel as a relic that allows you to spin it as a free action in every rest site and choose if you want that reward, removing the relic.
In the augmenter event you can fight the augmenter and take all three options if you win, the same idea goes for the totem that gives you three choices in act 1.
You can hire the three bandits from act 2, they will help you on your first turn against the act 3 boss.

That’s just a few simple ones, there are some events that last longer, kind of like an event chain, my favourite one is where you can pick between a useful relic that adds a shiv to your hand every turn, or a seemingly useless bird statue relic. If you take the bird statue, the next cultist you face will run from the fight, and if you come across the sacrificial dagger event in act 2, you can now select every option and the cultists will even allow you to kill a few of their members with that sacrificial dagger, stacking it up.

These events are awesome, like I said they’re definitely the highest quality of any act mod I’ve played, and it all fits into the world of slay the spire with the characters we know and love, or …hate.


Hexaghost is the third character you’ll unlock, concluding the act 1 boss trilogy. This guys main mechanic revolves around igniting the 6 ghostflames that surround him, just as the vanilla boss does. You ignite these by playing the card types required, each one having a different effect, then advancing to the next ghostflame next turn. The first ghostflame requires 2 attacks to apply 4 soulburn to a random enemy, we’ll talk about soulburn in a bit, the second flame requires two skills to deal 4 damage to two random enemies, the third flame is the same as the first, the fourth flame requires a power to gain 4 block and 1 strength, the fifth flame is the same as the first, and the final flame requires you to spend three energy, and will deal 4 damage to a random enemy for each ghostflame ignited, then extinguishes all of them, effectively starting the cycle over. There are a lot of ways to make igniting these ghostflames more worth it, the starting relic spirit brand already gives you 6 block the first time you ignite one each turn, the intensity buff given by a few cards will increase the damage, soulburn, or block applied by all of these ghostflames. There are a bunch of cards that will force-ignite the current flame, and retract or advance the flame to manipulate this mechanic, your starting deck has the float card that allows you to advance to the next flame, upgraded allows you to also retract. There’s also powers that buff these flames further like radiant flame applying soulburn to a random enemy whenever you ignite any ghostflame.
You’re probably wondering what soulburn is, I’ve mentioned it a few times, and your starting deck has this sear card that lets you apply 9 soulburn for 1 energy. It’s pretty simple, it’s just a stacking debuff that deals it’s stacks as unblockable damage on the third turn after applying it, it’s kind of like poison except you have to wait a few turns. There are a ton of cards that apply and manipulate soulburn, it’s definitely been the most succesful archetype for me when playing hexaghost. Cards like burning touch can apply a ton of this debuff, 10 soulburn and 10 additional soulburn if the enemy already has some. Pair this up with powers like extra crispy making all soulburn apply 2 extra stacks, and then something like heat crush that deals 30 plus the soulburn stacks as frontloaded damage, or even living bomb that makes the next soulburn detonation on an enemy apply the damage to every enemy.
Since the hexaghost is a ghost, ethereal cards are a big theme for this class. You’ll come across a lot of cards with ethereal, some even having afterlife which means it’ll automatically play whenever exhausted. Common cards like this are specter’s wail that deals AOE damage, and hexaguard gaining block and drawing a card. A lot of these ethereal cards are pretty strong, but you can support them with synergistic cards like ghost lash or shield dealing damage or blocking an extra time if you have an ethereal card in your hand, or paranormal form making you deal 7 damage to a random enemy when you play an ethereal card.
There are 6 different seal cards you can collect, each being pretty lackluster powers that give you a bonus reward at the end of the combat, like the second seal giving you additional money, the fourth seal giving you an additional potion, and the sixth seal giving you an additional seal for the cost of 3 energy. These all have ethereal, aren’t that strong, and clog up your deck, but if you manage to collect all 6 seals and play every single on in one combat, you’ll be rewarded with a pretty awesome relic that I won’t show you because I want you to try to find it yourself!
The hexaghost is pretty cool, definitely the most complicated of all the classes, it took me a while to fully understand these mechanics, but hopefully me explaining this stuff can help other people beat up these spire slayers with this class.

SHOPKEEPER & Colourless Cards

Whenever you visit the shop, the shopkeeper will not want to sell to you, but instead fight you. He always goes through the same cycle of blocking, attacking, stealing souls, and then fleeing. After he flees, the heart will appear through a portal and then you can buy stuff from it. Every time you face this shopkeeper, he’ll retain the HP you left him at last fight, so it is very possible to kill him before he flees if you visit him a few times, and he’ll reward you with all the souls he has stolen from you plus a bunch more.
This is a neat little mechanic.

On the topic of shopkeepers, let’s talk about the colorless card pool that now have a bunch of new cards added, since the shop is the main place you’ll be finding thee. Most of these are boss cards, you can only play one boss card per turn, but they are all really powerful. Each of the vanilla bosses has a few cards in this pool, like the awaken power making you revive with 10 hp when you die, chrono boost gaining you 2 strength every 12 cards you play, deca’s protection and donu’s power giving you 1 dexterity or 1 strength and adding a random power or attack to your hand to cost 0 this turn. The biggest deal in colorless cards would be study the spire, a three energy power that adds a random boss card to your hand that costs 0 for the next three turns. There are a lot of new colorless cards, this makes things like colorless potions and the event that gives colorless cards a lot more exciting.


The playable snecko is the fourth class, seperate from the three act 1 bosses and in it’s own little bubble. It’s definitely the class I played the least of, so will therefore be speaking the least about, but he’s basically all about gambling. His starter deck comes with two unknown cards, these cards roll into ANY random card from ANY class, including the vanilla classes and any modded classes you have installed, and keep that form the the rest of combat, rerolling into another one at the start of the next combat. You’ll come across a lot of different unknown cards that specify which type of cards they can roll into, like unknown x cost cards, unknown power cards, unknown class specific cards, unknown vulnerable cards, and so much more. Most of the snecko’s card pool consists of these unknown cards, and the starter relic snecko soul will increase your max hp by one every time you add an unknown card to your deck, so it’d be hard to play a snecko run and not take a bunch of unknown cards. The other starting cards are tail whip, costing 2 energy to deal 10 damage and apply 0-3 weak and vulnerable, and snek bite, dealing some damage and muddling the highest cost card in your hand. When you muddle a card, you randomize the cost between 0 and 3, similar to the snecko eye relic. Muddle is a big mechanic for this class, there are cards like soul cleanse that will muddle your hand without allowing anything to cost 3, and snake rake simply damaging, drawing, and muddling the highest cost card. The Snekproof keyword means that the card cannot be muddled, which can be a really good thing if that card is already cheap like like snake sap dealing low damage buy gaining 0-4 energy back for a 1 energy cost.
There’s not really much else to this class, almost every single card that isn’t an unknown card relies heavily and gambling with potentially large rolls like dice crush starting at 1-14 damage but increasing the max damage by 1 for every unknown card in your deck, and dice block doing the same thing but with block.
To me at least, the main part of this mod involves the first 3 classes that are amazingly designed and well balanced, then this snecko is here just to have fun with a lot of these new mechanics added.


You work for the heart, so it wouldn’t make sense for you to go fight the heart in act 4 like in vanilla, right? This mod completely reworks act 4 into a new challenging end game fight. If you want to discover this all for yourself, I’d highly recommend to finish the video here and go play, I’ve already talked about all of the super cool things I wanted to.
We still have the format of super hard elite, shop, campfire, and then super hard endgame boss.
In this case, the elite is the shopkeeper who has been resurrected and is now much stronger. His deck consists of colorless cards, he starts turn one with a panic button and a bomb, which are both problems especially considering his calipers. He’ll then go through all of his cards, doing what colorless cards do best, the main worry with him is that bomb dealing 40 damage on turn 3 then scaling with sadistic nature and applying a lot of debuffs to you every turn.

After that, you’ll face off against the real boss, Neow. He has 300 health, but don’t let that fool you, because his god of life passive will not let him die, at least not the first time. He starts the combat applying four really annoying new curses to you, and then cycles through really heavy hitting attacks and blocking and gaining 6 strength.
When you manage to get him to 0 hp, he’ll retreat and revive the act 1 boss you fought earlier this run with the same set of relics and cards, but now with the vastly increased HP of the neow’s blessing relic. Once you kill this boss, you fight neow again with the curse re-applied to you, and then he’ll revive the act 2 boss. You go through this again until the act 3 boss, which gives you the special cards from gaining the keys back. When you beat this third boss, you are then able to kill neow once and for all. Without a strong deck, this fight can go on for very long which is a problem because neow is constantly gaining strength from his block move, and the three bosses you face will often have very strong scaling from things like strength, orb summoning, accuracy, mantra, and whatever version you’ll face has.
I really like the idea of this fourth act, it’s an awesome conclusion, but I do think it’s a little too easy at the moment, at least on the early ascension levels. This is fine because this mod is constantly being updated and balanced, I’ve found many times that some cards aren’t the same that they were the previous day, a lot of work is going into balancing this mod.


In conclusion, this mod is awesome. You can tell that a lot of work and passion has went into this, the list of creators is long and they all deserve props for creating such an awesome experience. The classes are all awesome, the boss fights are really well done, and the events bring it to the next level. I’d highly suggest you try out this mod if you haven’t been convinced already, it’s really cool that all of the hallway and elite fights stay the same so you don’t have to relearn an entirely new game, the only thing you’d have to re-learn are the boss fights and these new classes.
If you like content like this, and are interested in the slay the spire modding scene then feel free to subscribe, I love making videos like this and giving these mods the publicity they deserve so much.

Thank you for watching, have a great day.

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