Soulcage



The Soulcage was a magical lantern with the power to compel the soul of any being. It was granted to Sylvanas Windrunner by Helya as part of the secret deal they made in Helheim. Sylvanas used it in an attempt to subjugate Eyir in the vault of Skold-Ashil, but Genn Greymane freed her when he smashed the lantern. Afterwards, the adventurer received a shard of the lantern to empower their artifact weapon.

Speculation
Judging by its abilities and its pale blue glow, as well as Helya being an ally of the Jailer, the Soulcage may have been an instrument of Domination magic.