Last updated: 10 August 2026

Slay the Spire 2 Act 3 Boss Guide Beat Every Boss

Act 3 is where Slay the Spire 2 runs succeed or fail. The enemies hit harder, the patterns are more complex, and the three possible boss encounters — The Queen, Test Subject, and Aeonglass — each test a different aspect of your build. This guide covers all three bosses' attack patterns, their most dangerous windows, and the strategies that work for every character. Note that Slay the Spire 2 is in Early Access and boss mechanics can change with patches — cross-check the official wiki for your current version.

01Act 3 General Preparation

Before engaging any Act 3 boss, make sure you've completed these steps during Act 3's map:

• Have at least one upgraded key card. Upgraded Defend (8 Block instead of 5), upgraded Whirlwind, or upgraded Demon Form all make a meaningful difference in boss fights. If you haven't upgraded your most important card, prioritise a campfire Smith over a heal unless your HP is critical.
• Carry at least two potions. Potions are most impactful against bosses. An Explosive Potion (deal 10 damage to all enemies), a Strength Potion (+5 Strength for the combat), or a Block Potion can decide a fight. Don't enter Act 3 with an empty potion slot.
• Understand your deck's kill speed. If you can deal 100+ damage in a single turn with your combo, prioritise offensive potions. If your deck is slow and grinds through Block loops, defensive potions and HP are more valuable.
• Check for Relic synergies with the boss patterns. Some relics (like Bag of Marbles, which applies Vulnerable at combat start) have outsized value against high-HP bosses since the Vulnerable lasts many turns.

02Boss 1: Test Subject

Test Subject is a three-phase revival boss. It refuses to die at 0 HP in phases 1 and 2 — instead it immediately comes back with more HP and a new modifier. Each phase transition clears all its status effects, so debuffs you applied do not carry over.

Phase 1 (100 HP) — Enrage:

• Test Subject gains 2 Strength every time you play a Skill card. This escalates fast — three Skills in a single fight can push it to 6 extra Strength on all attacks.
• Counter: In Phase 1, play Attacks and Powers instead of Skills to slow Strength accumulation. Block is less critical here because Phase 1 attacks (Bite: 20 damage; Skull Bash: 14 + Vulnerable) are manageable without investing heavily in Skills.
• If your deck is heavy on Skills (Exhaust builds especially), consider potions or aggressive burst to kill Phase 1 quickly before Enrage stacks up.

Phase 2 (200 HP) — Painful Stabs:

• Any damage you take that is not blocked shuffles a Wound into your discard pile. Wounds are unplayable cards that clog your hand and deck for the remainder of the fight.
• Counter: Block everything in Phase 2. This is the phase where defensive cards are most valuable — an unblocked hit of 10 damage is less threatening than the deck pollution from Wounds. Multi-Claw (10 damage ×3, with the hit count increasing each use) is the dangerous attack; Block before it fires.
• Strength Potions and burst damage are useful here to end Phase 2 before Multi-Claw starts adding hits.

Phase 3 (300 HP) — Nemesis (Intangible):

• At the end of every other turn, Test Subject gains Intangible (all damage reduced to 1 until your next turn). Big Pounce deals 45 damage; Burning Growl shuffles Burn cards into your deck and grants it additional Strength.
• Counter: Save your highest-damage burst (Heavy Blade combos, Doom execution threshold) for turns when Intangible is not active. On Intangible turns, focus on building Block and playing zero-cost cards. If you have multi-hit attacks, even 1 damage per hit adds up across many hits on Intangible turns.

03Boss 2: The Queen

The Queen is found in the Glory area and fights alongside her minion, the Torch Head Amalgam. This is the most mechanically complex of the three Act 3 bosses because it is a two-enemy fight with a powerful restriction mechanic.

The Queen's Mechanics:

• Chains of Binding — The Queen's signature ability restricts you to playing only one of the first three cards drawn on that turn. This is devastating if it hits a turn where you needed all three cards to set up Block or deal lethal damage. Plan around this by having 0-cost cards that don't 'waste' a restricted slot.
• Frail and Weak debuffs — The Queen applies 99-turn versions of Frail (reduces Block gained by 25%) and Weak (reduces Attack damage dealt by 25%). These last effectively forever and change the math on your Block and damage calculations for the rest of the fight.
• Torch Head Amalgam — The Queen's minion acts as the primary damage dealer, hitting up to three times per turn. While the Queen applies debuffs and restrictions, the Amalgam is what actually kills you.

Counter strategy:

• Kill the Torch Head Amalgam first. The Amalgam deals more immediate damage than The Queen and has lower HP. Focusing it down removes the multi-hit pressure and lets you manage The Queen's debuffs at your own pace.
• Recalculate your Block needs after Frail is applied. If you normally need 20 Block to survive a hit and Frail reduces your Block by 25%, you now need roughly 27 Block sources to reach the same protection. Adjust how many Defend and Block cards you play accordingly.
• Weak reduces your Attack damage by 25%. If your kill plan requires dealing 50 damage in one turn, you now need sources that produce ~67 damage to achieve the same result. Either plan to deal more or rely on Doom (which executes rather than dealing direct damage) or Juggernaut (which converts Block into damage, also reduced by Weak).
• Save potions for the Chains of Binding turn. When you know Chains of Binding is coming (it's visible in The Queen's intent icons), hold a potion to compensate for the restricted hand. An Explosive Potion or Strength Potion used on that turn still deals full value even with the card restriction.
• For Necrobinder players: Doom is not affected by Weak (Doom is not an Attack), making this fight particularly strong for Doom builds. Apply Doom to both enemies and let the Amalgam execute first.

04Boss 3: Aeonglass

Aeonglass is the newest Act 3 boss, added as part of an ongoing rework of the Act 3 boss pool during Early Access. It has 512 HP and no phase transitions — instead it uses escalating Wither pressure to punish long fights. Always verify the current pattern in the official wiki for your game version.

Core mechanics:

• Withering Presence (passive) — every 6 cards you play, Aeonglass adds a Wither card to your hand. Wither is unplayable and deals 3 damage to you at the end of your turn if it remains in hand. The Wither count builds as the fight extends.
• Repeating cycle: Ebb → Eye Lasers → Increasing Intensity — Ebb deals 22 damage and gains 33 Block (Patch v0.109.0, July 17, 2026, reduced Ebb's damage from its earlier 26). Eye Lasers deals 11 damage twice. Increasing Intensity shuffles a Wither into your discard pile, grants Aeonglass Strength, and upgrades all existing Withers (each upgraded Wither deals more damage at turn end).
• Artifact (3 stacks) — Aeonglass blocks the first three debuffs applied to it. Account for this when planning Vulnerable or Weak setups.

Counter strategies:

• Kill fast to minimise Wither accumulation. Each use of Increasing Intensity upgrades Withers already in your deck — the longer the fight, the more unplayable damage cards you're holding. Burst builds (Strength + Heavy Blade, Doom execution) benefit most here.
• On Ebb turns, don't waste high-damage cards. Ebb grants 33 Block — plan your burst for Eye Lasers or Increasing Intensity turns when Aeonglass has no Block buffer.
• Exhaust builds manage Withers well. Cards that Exhaust cards from hand (or Sentinel/Feel No Pain triggers) can clear Withers before they deal end-of-turn damage. Corruption + Dark Embrace + Feel No Pain generates sustained Block against the predictable single-target cycle.

05Universal Act 3 Boss Tips

• Read the intent before every turn. All Act 3 bosses telegraph their next action through the icon above them. Never play a full offensive turn into a boss that is about to deal 40 damage — always check and Block appropriately first.
• Use your highest-value potions here, not earlier. A Strength Potion (+5 Strength for the combat) combined with Heavy Blade can deal 60–80 damage in one turn. That's often enough to swing a boss fight.
• Enter with at least 60% max HP if possible. Most Act 3 bosses can deal 30–50 damage on their first turn if your Block is insufficient. Entering at 40 HP against a 30-damage opener leaves no room for error.
• Know which boss you're facing before entering. The boss encounter is visible on the map before you commit to the path. If you have a choice of routes, look at what boss each path leads to and pick the one your deck is better equipped for.
• Doom is effective in Act 3 but requires setup. The execution fires when an enemy's current HP is at or below your total Doom stacks (an absolute comparison, not a percentage). Chip Act 3 bosses with regular damage first to lower their HP, then set the Doom threshold for the final kill window. Doom bypasses Block entirely and ignores Metallicize, making it valuable regardless of how much Block the boss has built.