The Bugbear Mothership is a special zone that's only available to characters who are ascended under the Bugbear Invasion special challenge path.
It can be found where the Lair of The Naughty Sorceress normally is, because the entire Mothership crushed it upon landing.
Sneaking in allows access to a number of subzones:
In order to gain access to a level on the ship, you need to reroute power back to the elevators from the rooms on the previous level.
In order to access rooms on a level, you must find enough biometric data for your key-o-tron in order to unlock the door.
Trying to go here after freeing the King gives this message:
- This is not the place you're looking for.