Reaper's End Scarecrow Spectre

"Harvest's reaping brings out all sorts of ill-omened beings. I avoid that nonsense by looking like a scarecrow spectre animated by amethyst flames. An illusion, yes, but yours if you can handle the terror you'll inflict."—Othreloth Hlaalo, Illusionist

Source: in-game description.

This model is used for the Reaper's End Scarecrow Spectre polymorph.