This creature is both extremely rare and extremely dangerous. Contended Apparitions occur when two or more specters inhabit a body of a living medium to large creature, and remain there. The creature becomes maddened, lashing out at everything nearby with insatiable blood-lust. Even after the creature dies of the strain, the remaining body and specters possessing it are extremely dangerous.
(There's an awesome mini-boss for you. The moment the party thinks they've defeated it, it can rise from it's collapsed position, even more furious then before, and requiring a second butt-kicking. It would (of course) be awesome if it totally ignored some fatal injury in it's second state as well, like if it were missing half it's face.)