Jewel of Hellfire

Source
This item is found on an orc skeleton on the northern coast of Tanaan Jungle, below the cliffs of the Ruins of Kra'nak.

Using the toy turns the user into a man'ari eredar (with a glowing jewel sticking out of their forehead) for 5 minutes.

Trivia
This item is a possible reference to the Soulstones in the Diablo franchise.