Soul (Warcraft III)

A Soul Gem is an artifact used to capture and imprison a hero's soul. Thrall used such a gem, given to him by Jaina Proudmoore to capture Grom Hellscream in order to rid him of the demonic curse that had befallen him upon drinking Mannoroth's blood.

Source
Thrall uses this in By Demons Be Driven, chapter 8 of the Warcraft III orc campaign, The Invasion of Kalimdor.

In the RPG
The Soul Gem is a minor artifact with a single, potentially devastating power. The jewel appears to be any of a number of other common baubles found in treasure hordes across the land. Only when the quarter-inch blue jewel is exposed to a magic detecting spell does its strong power reveal itself. The gem's true nature cannot be discovered until it is dipped in blood from an intelligent living creature sacrificed for this purpose. When this ceremony is completed, the jewel swells to 10 times its normal size and begins to glow with a pulsing violet light. The gem can now be used to entrap any one living creature with but a single command word. The target must be within 60 feet for this power to work. The creature within can be released by a second command word from the creature that entrapped it, or held forever in that state. No other known magic can release a trapped individual. The soul within can also be used for a variety of vile demonic purposes known only to a few tortured sages and elders of the Burning Legion. When the jewel is emptied, by whatever means, it returns to its original form. The Soul Gem cannot hold more than one creature at a time and it may not trap another while one remains within.