I can't recall if I reposted this one, so I'll just stick it here.
Doom siren: A creature who, like the sirens of Greek myth, sings men to their deaths. Unlike those sirens, though, they live in the dungeon, and so instead of singing lovely songs that cause men to sink their ships trying to reach the seemingly loveliest voice, they sing songs of such deep sadness and pain that those who hear it are driven to suicidal acts. A kobold carrying poison for his hunting drinks it in despair; a kamikaze dwarf becomes a fine mist at the seeming loss of his god's favor; a dark mage electrocutes himself trying to bring his power back. Those wishing to slay such monstrous demons may stop up their ears, though fitting items be few and far between; braver men may attempt to fight through the magics of the song, but the doom siren is not a weak maid to be killed easily, and all the while such a man's body would be prone to turn treacherous such that whatever weapon used is turned against him.
translation: this monster has the ability to make players and monsters do things that lead to death, like hitting themselves with their own weapons, setting off known nearby (1 square distant) traps, blasting themselves with wands, etc. (It activates the same way as the enner beast's ability.) I have heard this would be really hard to program and I don't doubt it, but you asked for ideas......