Impostrel

Description:

An impostrel is a shapeshifter with a vaguely humanoid form that that preys on its foes by separating them from their group, tricking them into thinking it is one of them, and then attacking with surprise. In it's undisguised form, it as a frightening looking mass of twisted but featureless pink flesh with constantly shifting folds and patterns slowly twisting across its 'skin'.

Special:

An impostrel is a cautious but cunning predator that will use its spells first to separate individuals, and then to narrow in on a specific target by impersonating an ally, getting close, and attempting to launch a decisive attack using its special skills.

Impostrels are weak and will never engage in a stand-up fight. If they are unable to subdue prey with a decisive surprise attack, they will retreat and look for easier prey. They would never choose to attempt to wipe out an entire party of adventurers, as a single humanoid-sized victim can feed them for many days, and more than one will rot before they can get to it.