Eye Tyrant said:Should the Aboleth be the Mastermind (BBEG) or just a coincidence? How did the fortress come to life? Did the the Aboleth cause it or was it an effect from the grear cataclysm at the end of the last age? Are the Undead fallen warriors that defended the fortress during the war, or enemies, or both?
The fortress came to life as part of a spell researched/created by the original owners. The spell summoned an earth elemental to "possess" the fortress. The command was "anyone not standing in this room with me right now is an invader. Destroy them."
The people who owned the fortress were desperate to defend it, facing a simultaneous attack from armies on the surface, and those who were in service of the Aboleth and trying to infiltrate it from below. The aboleth still lurks in the depths, it's servants are so far unable to defeat the elemental.
I don't know about the source of undead. My elemental idea was an attempt to avoid undead because I think they can be overused.