Hym

The hym is a rare and dangerous specter that only latches onto people burdened by guilt over a terrible deed. Innocent people can’t see it at all. To its victim it appears as a tall, shadowy, humanlike silhouette with long, sharp claws, though it rarely uses them directly. Instead it feeds on guilt: it whispers only to its host, conjures nightmares and hideous visions drawn from their worst fears, and pushes them toward violence and self-harm until they lose their mind.

When it does fight, the hym moves slowly, but its ethereal claws have long reach and drain life on contact. After taking enough damage it can vanish and reappear elsewhere in the room.

In The Witcher 3 you meet it in Possession, a secondary quest on Spikeroog in Skellige. Cerys an Craite believes Jarl Udalryk is haunted by his drowned brother Aki and searches his old family home for the sword Brokvar. Geralt eventually realizes the real culprit is a hym feeding on Udalryk’s guilt. There are two ways to end it: trust Cerys and her plan to trick the hym into a new host who discovers he did nothing wrong, or take “the witcher’s way” and fight it in the haunted house with Udalryk present. Only the fight drops loot.