The soul beckoner is an undead creature, similar to a wraith, which drains the life force of its victims. They look more like shadows though, and are mistaken for them more often than not. As this creature drains the life energy of its victims, it takes on aspects of their appearance and changes, becoming larger and more solid. The soul beckoner is more in tune with its previous form than a typical wraith, and therefore has a stronger tie to the Material plane.
A soul beckoner lies in wait for prey to come in range of its whisper attack to lure them into melee range. It then attacks with its bony claws, draining the energy of the living and adding to its own strength. Each claw attack can drain energy with a successful attack; therefore a soul beckoner can drain up to two levels per round.
It's a little bland, but nothing stands out as really bad. Any ideas?