Undead. Immune to offensive psionic power cards and offensive spells. Psionicists defeated by the Caller are sent to the Abyss.
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TSR
Wizards of the Coast
The Caller in Darkness is a pretty good champion. His power is not great, but his high level makes him a good champion to play with the rule card Negative Planar Energy (73/NS).
Unless you are building an undead psionicist deck, the best use for Caller in the Darkness is in a deck themed around the Abyss. Add him to a deck loaded with cards that send people to the Abyss like Mindkiller (56/UD) and Takhisis's Abyssal Gateway (13/DL Chase), then use the Bottomless Horror (54/DU) to remove them from the game.
The Caller in Darkness is an excellent choice for any undead deck using psionic power cards, since so few undead are able to use psionic power cards. You may also encounter this champion in a Dark Sun world deck based on just its immunities and high level.