Soul Eater

The Soul Eater is one of those nonstandard monsters from para-Dungeons & Dragons, in this case David "Zeb" Cook's adventure module X4: Master of the Desert Nomads. It is a cloud of amorphous shadow with luminescent pale clawed arms... reaching for you.

Not unlike undeads' energy drain this one sucks your Wisdom, albeit temporarily. When all the WIS is taken, your Soul is Eaten and you ain't coming back. At least: not by Expert-level Raise Dead or Reincarnation. (Later Tome of Horrors will allow Divine intervention. We suppose Basic Dungeons & Dragons might square this circle by Resurrection.)

The bad news: you, the victim, cannot kill this monstrosity in the Prime Material. The good news: you can defeat it; and if/when you do, it will spin around to attack the asshole who had summoned it upon you. (Not that this will necessarily help on account the summoner is likely some massively overpowered cleric... like Hosadus.)

The Soul Eater is one of those critters like the Nagpa and Tabi but more so, as something alien to whatever the party had experienced up to now in more "Western" environments. (Southeastern, if Mystara.) Frank Mentzer deemed it too-weird for the Companion Set. We could understand Mentzer's decision here if he hadn't included the next module's malfera for reasons.

So the Soul Eater has always skirted the edge of the monster-collections. It made it to the BXCMI Creature Catalogue; also, Tome of Horrors.