Slay the Spire 2 Biome Guide: Overgrowth vs Underdocks

Two Paths Into the Spire

Act 1 in Slay the Spire 2 doesn’t work like you’d expect. Instead of a single set of enemies and events, the game randomly assigns one of two biomes: Overgrowth or Underdocks. Each has completely different enemies, elites, events, and bosses. The experience of Act 1 changes dramatically depending on which biome you roll.

Act 2 is always the Hive. Act 3 is always Glory. Mega Crit has hinted at more biome options for later acts on their roadmap, but for now, Act 1 is where the branching happens.

Understanding both biomes — their threats, their rewards, their events — is how you stop getting blindsided by Act 1. Let’s break them down.

Overgrowth

The Vibe

Overgrowth is a verdant forest. The visual design leans into tangled vines, ancient trees, and overgrown ruins. It feels alive and hostile at the same time, like nature decided it doesn’t want you here.

Enemies

Overgrowth enemies tend to hit with moderate damage across multiple attacks. Expect multi-hit patterns and enemies that buff each other. The standard fights are manageable if you have decent AoE damage. Single-target heavy decks struggle here because Overgrowth loves throwing groups of 3-4 smaller enemies at you.

The easy pool (first 3 fights) contains genuinely gentle encounters. Don’t let that fool you — the hard pool enemies ramp up fast with stacking buffs and increasing attack counts.

Events

Overgrowth has roughly 12 events. They tend to offer trade-offs involving HP, gold, and card manipulation. Several events can transform or remove cards, which is valuable in early Act 1 when your deck is full of starter garbage.

Remember: every event in Slay the Spire 2 has real consequences on every option. There are no free plays. That said, Overgrowth events trend toward being slightly more generous than Underdocks. The penalties are usually HP-based, and HP is the easiest resource to recover through rest sites and relics.

Elites

Overgrowth elites hit hard but predictably. They telegraph their big attacks, giving you a turn to prepare Block. If your deck has any defensive answers, you can handle them. The danger is going in underpowered — if you can’t kill them within 4-5 turns, their damage output starts outpacing your ability to block.

Bosses

Overgrowth bosses are aggressive. They test your ability to output damage quickly while maintaining enough defense to survive burst turns. Ironclad and Necrobinder tend to have an easier time here because their kits naturally lean into the “race them down” approach these bosses demand.

Best Characters for Overgrowth

Ironclad — His AoE attacks handle Overgrowth’s multi-enemy fights. Burning Blood heals the chip damage. Strength scaling races down bosses. This is his biome.

Necrobinder — Doom stacks land well against Overgrowth enemies because fights tend to be longer multi-target affairs, giving stacks time to build. Osty helps with crowd control.

Silent — Works fine. Poison handles multi-enemy groups through AoE application. Not her best biome, but not a problem either.

Underdocks

The Vibe

A flooded harbor district. Dark water, rotting piers, smuggler dens. The enemies feel scrappier and meaner, like everything down here is fighting dirty.

Enemies

Underdocks enemies lean toward fewer, tougher individual creatures. Where Overgrowth throws groups at you, Underdocks prefers 1-2 enemies that hit like trucks and have annoying abilities — debuffs, Block penetration, HP shields. Single-target damage is king here.

The easy pool is still forgiving, but Underdocks hard pool enemies can ruin your day with debuff stacking. Weak and Frail applied by enemies feel worse than raw damage because they reduce your ability to fight back.

Events

Underdocks has roughly 10 events, a couple fewer than Overgrowth. They tend toward risk-reward gambles involving gold and relics. Some events offer relic rewards for significant HP costs or permanent downsides. The trade-offs feel sharper than Overgrowth — higher highs, lower lows.

Several Underdocks events interact with your gold total. Having more gold opens better options. This makes early monster fights (which generate gold) more valuable in Underdocks than in Overgrowth.

Elites

Underdocks elites are tanky. They have more HP and more defensive abilities than Overgrowth elites. Fights take longer, which means you take more total damage even if per-turn damage is lower. Sustained damage output matters more than burst here.

Poison builds shine against Underdocks elites because poison ignores enemy Block and scales over long fights.

Bosses

Underdocks bosses are attrition fights. They test your ability to sustain damage and defense over many turns rather than racing to a quick kill. Decks that peak on turn 1 and fall off will struggle. You need builds that maintain or increase power over time.

Best Characters for Underdocks

Silent — Poison is tailor-made for Underdocks. It stacks through enemy defenses, handles tanky elites, and scales over the long boss fights. Weak application reduces the sustained incoming damage. Silent thrives here.

Defect — Frost orbs provide the sustained Block that Underdocks’ long fights demand. Lightning orbs chip away at tanky enemies over time. Defect’s scaling nature matches the biome’s pace perfectly.

Regent — Star banking works well in longer fights. She builds resources during safe turns and unleashes them when the enemy is vulnerable. Underdocks gives her the time she needs to set up.

Biome-Dependent Strategy Adjustments

You can’t choose your biome, but you can adjust your Act 1 strategy once you see which one you got.

In Overgrowth:

• Prioritize AoE damage cards in early card rewards
• Take more aggressive paths with extra elites (Overgrowth elites are more forgiving)
• Use events for card removal — Overgrowth events offer this more frequently
• Block can be lighter since fights end faster

In Underdocks:

• Prioritize single-target damage and sustained scaling
• Be more cautious with elite fights — they take longer and cost more HP
• Hoard gold early because Underdocks events reward having money
• Invest in Block and debuff cards since fights drag on

The Biome Doesn’t Decide Your Run

Bad players blame the biome. Good players adapt to it.

Rolling Underdocks on Ironclad isn’t a death sentence. It means you adjust your card priorities toward Strength scaling (which handles single-target damage) and avoid leaning too hard into AoE that won’t help against 1-2 tanky enemies. Rolling Overgrowth on Silent means picking up AoE poison application early instead of single-target stack-and-wait cards.

The biome changes your first few card and path decisions. It doesn’t change your character’s core identity. Adapt early, commit to the right plan, and Act 1 is completely manageable regardless of which biome the game hands you.

What’s Coming

Mega Crit’s roadmap mentions additional biome options for Act 2 and Act 3. Right now Hive and Glory are fixed, but expect the same branching structure to expand as Early Access progresses. When that happens, biome-specific knowledge becomes twice as important. Build the habit of reading the biome now so you’re ready when the map gets more complex.