Soulstealers

All Daemons can potentially possess a mortal, replacing his soul with their essence. However, few Daemons do, perhaps because the circumstances aren’t right or more likely because the opportunity just doesn’t present itself. But there are some Daemons who exist only to spread corruption and to do so they seize the bodies of their victims and drive them to commit unspeakable acts of evil. Enter the Soulstealer.

A Soulstealer is a smoky and insubstantial being that looks like a shadow cast upon the wall. It moves with a silent grace, blending in with the natural darkness, disappearing if it must. When it finds a suitable victim it slips in through his nostrils and hides in the recesses of his mind, subtly guiding his host onto the path of damnation. Soulstealers target those individuals who are already at the brink of corruption. They use their influence to push them over. Failing that, they might invade a child or some other easily influenced mortal out of spite alone.

Some Black Magisters summon Soulstealers for the express purpose of corrupting a troublesome opponent believing that somehow if a Daemon infests the individual the barricade to their goals will be cleared. In most cases, such efforts backfire, either because the Daemon fails to penetrate the mortal's defences or because the Daemon has plans of his own, betraying his summoner at the first opportunity.

Source

 * Warhammer Fantasy RPG 2nd ED -- Tome of Corruption: "Chaos Touched Monsters" -- pg. 7