Slay the Spire 2 Alternate Acts Guide: Every Biome Variant and Strategy Differences
Complete guide to alternate acts in Slay the Spire 2 covering all biome variants, different enemies and elites, and which deck types each variant favors.
What Are Alternate Acts?
Every act in Slay the Spire 2 was designed to have two biome variants. When you start a run, the game picks one variant per act and locks it in for that run. Different enemies, different elites, different bosses, different events. It’s not a cosmetic swap. The entire combat landscape changes.
Right now, Act 1 is the only act with both variants live. You’ll get either Overgrowth or Underdocks, chosen randomly. Act 2 runs through The Hive and Act 3 through Glory, both with a single biome for now. Mega Crit has confirmed alternate biomes for Acts 2 and 3 on their Early Access roadmap, and some of that structural groundwork shipped with patch v0.102.0.
I think this system is the single biggest improvement over the original game. The first Slay the Spire had one enemy pool per act, and after 50 runs you’d memorized every possible fight. Alternate acts mean your Act 1 experience can be wildly different run to run, and once the full set is live, that variety will extend through the whole climb.
How to Unlock Alternate Acts
Unlocks are tied to the Epochs system. New players start with Overgrowth as their only Act 1 option. You won’t see Underdocks until you’ve completed a full Act 1 run with any character.
Here’s the unlock timeline:
- Overgrowth (Act 1) — Available from the start
- Underdocks (Act 1) — Unlocks after your first Act 1 completion
- Future Act 2 alternate — Expected during Early Access (not yet live)
- Future Act 3 alternate — Expected during Early Access (not yet live)
The unlock happens through your total accumulated score on the Timeline. You don’t need to do anything special — just play the game, perform reasonably well, and the Timeline nodes will open. Once Underdocks is unlocked, the game randomly assigns either biome at the start of every run. You cannot choose which one you get.
Act 1: Overgrowth vs. Underdocks
This is where alternate acts shine right now. These two biomes share zero enemy types. The strategic demands are completely different.
Overgrowth
Overgrowth is the forest biome. Tangled vines, ancient trees, overgrown ruins. It’s the gentler of the two options, and I’d argue it’s the better learning environment.
Enemies lean toward multi-hit attacks and group fights. Expect Leaf Slimes, Phrog Parasites with their Wriggler companions, and Myte-family mobs that stack Poison. The easy pool in the first three fights is genuinely easy here. But the hard pool ramps up fast with enemies that buff each other’s attack counts.
Elites include Bygone Effigy, Byrdonis Phrog, and Parasite. These fights test your ability to handle sustained damage and enemy scaling. Parasite in particular punishes decks that can’t kill quickly.
Bosses rotate between Ceremonial Beast (252 HP, a 2-phase fight with shifting patterns) and The Kin. Ceremonial Beast is the signature encounter — it transforms between phases and demands you have an answer for each one. The Kin summons additional enemies throughout the fight, testing your ability to manage multiple threats while focusing damage.
What Overgrowth favors: AoE damage, multi-target attacks, decks that can handle groups of 3-4 smaller enemies. Single-target heavy builds struggle here because the forest keeps throwing packs at you.
Underdocks
Underdocks is the dark harbor biome. Flooded docks, eerie fog, sea creatures that should not exist. It hits harder than Overgrowth and introduces mechanics you won’t see anywhere else in Act 1.
Enemies are nautical nightmares. Haunted Ship (63 HP) literally steals cards from your hand. Noisebot applies debuffs. Inklings carry Slippery 9, which makes them dodge attacks. Sawtooth hits like a truck. These fights test different skills than Overgrowth — you need precision removal and debuff management rather than raw AoE.
Elites are Phantasmal Gardener, Skulking Colony, and Terror Eel. Terror Eel is the one that ends runs. It hits hard, it hits often, and if your deck doesn’t have strong defensive options by the time you reach it, you’re dead.
Bosses include Lagavulin Matriarch (sleeps for 3 turns then unleashes Soul Siphon), Soul Fysh (211 HP, floods your deck with Lure Afflictions), and Waterfall Giant (250 HP, has a Pressurize self-destruct mechanic). These are meaner than Overgrowth bosses across the board.
What Underdocks favors: Single-target burst damage, strong Block generation, and decks that can handle Affliction cards. Card manipulation (drawing, exhausting, discarding) becomes way more valuable here because enemies mess with your hand directly.
For a deeper breakdown of both biomes, check out the Biome Guide.
Act 2: The Hive (Single Biome)
Act 2 currently runs through The Hive, a dense insect kingdom. No alternate exists yet.
Elites are Decimillipede (a multi-segment enemy that revives dead segments), Entomancer (punishes repeated Attack plays by adding status cards to your deck), and Infested Prism. These fights are mechanically complex. Entomancer alone can ruin a deck that relies on high Attack counts — every swing pollutes your draws.
Bosses are Kaiser Crab, Knowledge Demon, and The Insatiable (321 HP, with an instant-death sand-pit mechanic). The Insatiable is one of the hardest Act 2 bosses in any deckbuilder I’ve played. If you’re not prepared for its sand-pit timer, it just kills you. No amount of HP saves you.
When the alternate arrives: Expect the second Act 2 biome to split the elite pool. Based on early data mining and Mega Crit’s design philosophy, one variant will likely emphasize Affliction-heavy encounters while the other leans toward raw damage brawls. The two Act 1 biomes follow exactly this pattern — Overgrowth is the “fair fight” biome and Underdocks is the “dirty tricks” biome.
Act 3: Glory (Single Biome)
Act 3 takes you to the top of the Spire through Glory, the endgame biome.
Elites are Knight Trio (a three-enemy fight that tests everything), Mecha Knight, and Soul Nexus. These are among the hardest non-boss fights in the game. Knight Trio in particular feels like a boss fight itself.
Bosses include Doormaker (489 HP, runs a 3-turn Affliction cycle of Hunger, Scrutiny, and Grasp), The Queen (400 HP, summons minions and tests raw sustained damage), and Test Subject. Every Act 3 boss is rated at maximum difficulty by the community. The only consistent answer to these fights is a deck with at least one infinite or near-infinite damage loop.
When the alternate arrives: Act 3’s second biome will likely shuffle which bosses appear in which variant. In Act 1, the boss pools are completely separate between Overgrowth and Underdocks. Expect the same split here — two bosses per biome, different elite pools, different event lineups.
Strategic Differences: How Biomes Shape Your Deck
The real strategy of alternate acts isn’t just knowing the enemies. It’s understanding how each biome filters your deck decisions throughout the run.
Early Game Adaptation
In Overgrowth, prioritize AoE. Cards that hit all enemies are premium picks in the first few shops and card rewards. If you see a card that deals damage to every enemy, grab it — the forest will throw groups at you all act long.
In Underdocks, prioritize single-target removal and Block generation. You need to kill specific threats fast (Haunted Ship before it steals your cards, Terror Eel before it snowballs). Wide-spread damage doesn’t help when one specific enemy is the threat.
Deck Archetypes by Biome
Strength/scaling decks do well in both biomes but for different reasons. In Overgrowth, scaling lets you handle the increasing enemy counts. In Underdocks, scaling helps you burst down high-priority targets.
Block-heavy decks are safer in Underdocks. The harbor enemies hit harder per attack but attack less frequently. Strong Block per card goes further here than in Overgrowth where you’re taking multiple smaller hits from multiple sources.
Exhaust/manipulation decks are stars in Underdocks. When enemies are adding junk to your deck (Lure Afflictions from Soul Fysh, Dazed cards from Act 2’s Entomancer), being able to exhaust those cards keeps your deck clean. In Overgrowth, exhaust is less necessary because enemies don’t pollute your deck as aggressively.
Poison/DoT decks have a natural home in Overgrowth where the Myte-family enemies already use Poison, meaning some events and card rewards lean into that archetype. Underdocks doesn’t support Poison as directly.
Choosing Your Path
You can’t pick your biome — the game assigns it randomly. But you can recognize which biome you’re in immediately (the visual and audio design are completely distinct) and adjust your early card picks accordingly.
Here’s my general rule: look at your first card reward after the tutorial fights. Ask yourself whether it helps against groups (Overgrowth) or single targets (Underdocks). If it does neither, it better be so generically strong that biome doesn’t matter.
For boss prep, check the Act 1 Boss Guide and the full Boss Strategies guide. Knowing which boss pool you’re facing changes your relic priorities and shop purchases for the entire act.
Looking Ahead
Mega Crit’s roadmap promises alternate biomes for Acts 2 and 3 during Early Access. When those drop, every run will have eight possible biome combinations across three acts. That’s a massive jump in variety.
I’ll update this guide as new alternate acts go live. For now, master the Overgrowth vs. Underdocks split — it’s the foundation for everything else that’s coming.