Soul Eater

Unholy Fiends, Soul Eaters are summoned through demonic rituals and will pursue a single target until it or the target is dead. If a Soul Eater kills a creature, it eats its soul and the creature cannot be resurrected. They are not particularly intelligent.
Soul Eaters vary in power based on what was provided during their summoning ritual. A given name results in a weak Soul Eater, while a true name, blood or a personal artifact can make for a much more powerful Soul Eater.
While a Soul Eater is attacking an enemy whose name it knows, the enemy can't properly see their surroundings, so a group of Soul Eaters can effectively reduce a party to single combat. Soul Eaters take normal damage from critical attacks, resist physical attacks, and are weak against holy damage.