End of Abyss Optional Bosses: Brood Keeper, Morgue Dweller & Rematch

Beyond the seven story fights, three optional encounters wait off the critical path. Each drops a Vault Key — the story bosses only yield five of the eight keys, so finishing the suit-mod collection in suit mods and energy cells requires all three of these hunts. None are missable; post-game keeps every shuttle live.

What you need first

These hunts assume endgame tools — don’t start before the Lower Levels:

• Explosive Charges — every route breaks a weak wall somewhere.
• Grappling Hook — required for the Reactor Pool drain and the Brood Keeper’s key hunt.
• Incinerator — required for the Morgue Dweller’s area key. Story pickup in walkthrough part 6.

Centipede Husk rematch — Reactor Pool

Drain the Reactor Pool and climb down: the Centipede Husk returns for round two in its emptied arena. The drain switch is at the end of a four-step Waterworks chain:

1. Waterworks > B3, room on the left — press the wall switch (needs the Grappling Hook).
2. Right side of the circular structure near the top of B3 — press the second switch to partially drain.
3. Pump console on the east side of B3 — activate it to drain that section.
4. Exit through the door left of the pump console, climb down the ladder, and follow the path north to a dead end — Explosive Charge the blockage wall.
5. Through the top-right door of B3, follow the path — an Energy Node door and an ambush guard the final switch, which drains the Reactor Pool.
6. Back to the Reactor, B3 — climb down into the empty pool for the rematch. Reward: Vault Key #6 and the silver “Don’t Come Back” trophy, plus the Underbelly Key in a locker down there (used for a Data Shard).

Brood Keeper — Ventilation Tunnels

A brood mother sealed in the Tunnels’ upper reaches — the route is a maintenance-key gauntlet:

1. Destroy the weak wall right of the easternmost entrance to the Ventilation Tunnels from Factory East (Explosive Charge).
2. Continue right until you reach the Tunnels’ F2 — circle the upper catwalk and pull down the ladder to F3.
3. Go all the way right on F3 and take the stairs up — an ambush guards the Ventilation Maintenance Key.
4. Back to the bottom left of F3: use the Grappling Hook to pull down the ladder, climb up, open the keyed door, and press the switch inside.
5. Return to F1 and climb the ladder in the circular area — the Brood Keeper waits at the top. Reward: Vault Key #7.

A Data Shard hides along this same route — the keyed door on north F3.

Morgue Dweller — Infirmary

The boss of the optional Infirmary area — and the only fight standing between you and the Particle Disruptor:

1. Infirmary Key: burn the Parasitic Spire in the Lower Levels F1 labs with the Incinerator.
2. Ride the lift right of the Habitation Sector Shuttle — the Infirmary is a short medical wing crawling with Infected Patients, Doctors, and Head Crawlers.
3. F1: grab the Restricted Access Keycard from the small room; open the door at the top of the right corridor down to B1.
4. The Morgue Dweller is on B1’s right side — a close-quarters grappler in tight mortuary corridors, so keep your roll lanes open. Reward: Vault Key #8.
5. After the fight, continue north and burn the top-right wall — the Particle Disruptor is inside. The Auto-Defibrillator Vault and the Infinite Health Boosters cheat code are on B1 too.

Completion payoffs

All three keys finish the eight-vault set for the “Dungeon Crawler” trophy, and each boss auto-logs its creature entry toward logbook completion. If the Fractured Being is still standing, the final boss guide is your next stop instead.