Peryton

A stag's head and antlers on the body of a great bird, and it throws the shadow of a man. It keeps that shadow until it has killed one, and then the shadow becomes its own, and after that it is only an animal and no longer dangerous in the way that matters. Each peryton kills exactly one person, once, ever. The horror of the thing is entirely in whose shadow it is currently wearing.

Stoop: piercing, melee 10 ft, +4 to hit, 2d8+3, made from height and aimed at the chest. Take the Heart: against a creature reduced below a quarter of its HP, no attack roll and no save. The target dies. The peryton takes the heart, and the human shadow it has been carrying becomes its own. It does this once and then it is finished, and it does not do it again in its life.

Wears a Shadow: it casts a human shadow until it has killed. A party that notices the shadow does not match the creature has been given the whole of the warning that exists. Spent: a peryton that has taken a heart loses its stoop, its interest and its menace, and becomes a large odd bird that flies away. Killing it before it kills is the only version of this that works.