The three reported optional-boss locations in End of Abyss are the drained Reactor Pool, the upper Ventilation Tunnels, and Infirmary B1. The Centipede fight is a rematch; the Brood Keeper and Morgue Dweller require unlocking routes into their areas. Guides call the Centipede encounter “Husk Centipede” or “Centipede Husk”; “Severed Husk Centipede” is not consistently used in those guides.
Optional boss locations at a glance
| Encounter | Location | Main access requirements reported by guides |
|---|---|---|
| Centipede rematch (“Husk Centipede” or “Centipede Husk”) | Drained Reactor Pool | Complete the Waterworks B3 drain sequence; return to the Reactor. |
| Brood Keeper | Upper Ventilation Tunnels | Enter from Factory East; use an Explosive Charge, obtain the Ventilation Maintenance Key, use the Grappling Hook, and activate a switch. |
| Morgue Dweller | Infirmary B1 | Unlock the Infirmary from the Lower Levels with the Incinerator; obtain a Restricted Access Keycard inside. |
These locations and routes are reported by third-party guides, not confirmed here by a primary developer or publisher source. The guides do not establish a recommended order or compare the encounters’ difficulty. [Optional-boss guide] [Vault Key guide]
Where to find the Centipede rematch
Drain the Reactor Pool
The rematch takes place in the Reactor Pool after it has been drained. The reported drain route is in Waterworks B3 and requires several switches, a pump console, and an Explosive Charge:
- Use the Grappling Hook to reach and operate a wall switch.
- Activate the switch near the circular structure.
- Operate the pump console on the east side.
- Use an Explosive Charge to clear the blocked wall.
- Follow the upper-right route to reach and activate the final switch.
Return to the Reactor
After the pool drains, return to the Reactor and climb down into the pool to trigger the encounter. Guides vary between “Husk Centipede” and “Centipede Husk”; the supplied name “Severed Husk Centipede” is not consistently confirmed by those sources. [Optional-boss guide] [Vault Key guide]
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Where to find the Brood Keeper
Get the Ventilation Maintenance Key
Enter the Ventilation Tunnels from Factory East. Near the easternmost entrance, destroy the weak wall with an Explosive Charge. Proceed through F2 to F3; the guide reports an ambush guarding the Ventilation Maintenance Key.
Open the route to the upper tunnels
- Return to the lower-left part of F3.
- Use the Grappling Hook to lower a ladder.
- Open the keyed door with the Ventilation Maintenance Key and activate the switch inside.
- Return to F1 and climb the ladder in the circular room to reach the Brood Keeper in the upper tunnels.
The Factory East entrance, key, and return route are also described in another boss guide. [Optional-boss guide] [Boss guide]
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Where to find the Morgue Dweller
Unlock the Infirmary
In the Lower Levels, burn a Parasitic Spire with the Incinerator to obtain the Infirmary Key. Then take the lift near the Habitation Sector shuttle to enter the Infirmary.
Reach Infirmary B1
On F1, collect a Restricted Access Keycard and use it to open the route down to B1. The Morgue Dweller is on the right side of B1. Other guides also place the encounter in the Infirmary and describe Lower Levels access as a prerequisite. [Optional-boss guide] [Vault Key guide] [Boss guide]
How the routes affect which boss to pursue first
The sources do not recommend an order or establish which fight is easiest. Use the area you have already unlocked and the tools you have available to choose a route:
- Reactor Pool: choose this if you can complete the Waterworks drain sequence, including its Grappling Hook and Explosive Charge steps.
- Brood Keeper: choose this if Factory East is accessible and you can reach the Ventilation Maintenance Key and switch route.
- Morgue Dweller: choose this if you have the Incinerator for the Lower Levels Parasitic Spire and can obtain the Infirmary keycard.
Reported Vault Key rewards
The optional-boss guide reports Vault Key #6 for the Centipede rematch, #7 for the Brood Keeper, and #8 for the Morgue Dweller. A separate guide describes these as the three additional Vault Key sources that complete an eight-key set. These rewards are third-party guide claims and are not confirmed here against a primary game source. [Optional-boss guide] [Vault Key guide]
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- The Assault Units, the Krohns, will advance towards Zone 1, and once there, they'll attack the Heroes on the wall and try to break into the Citadel
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