Palace of the dead

[Aeliott]

Something akin to the Tonberry Chaser from Wanderer's Palace. Something ridiculously deadly that can't be killed, randomly stalking the halls, and you never know when he's gonna show up. Perhaps in the middle of mapping a floor you hear a deep, distant roar, and the music gets more intense signifying that it's appeared - while everyone starts panicking and deciding what to do! I know this could make it a bit unfair, but they could also add some kind of method of stalling him via a pomander, or maybe luring him over a trap. Could take it a step further and make its immobilisation temporary, adding a dilemma such as "do we get the hell out of this floor, or still try and risk getting as much treasure as possible while it's disabled?".