Adumbrali (The Living Shadows) creatures that appear as shadows that dwell in the Abyss and send out Seekers to capture souls for them to torture and devour. They are so named in the Song of Yste . Cf. Old Ones.
These monsters, like the Beings of Xiclotl and the Death Vines, are more of the obscure lovecraftian horrors that alot may never had heard of. These beings as described in the story "The Abyss" are two-dimensional beings of shadow that can only move in horizantal movement and are known to hunt humans with cruel delight until slaying them outright. In crossover, they're alot like in the story but are considered a type of shadow elemental of the malicious variety and like in the story can teleport into different realms and can shoot out shadow-like tendrals that suck the engulfed victim's body fluids, leaving nothing but a dryed up corpse once its done.
The Adumbrate were created by Robert A. W. Lowndes and expanded by Chaosium.