The Skull of Gul'dan

Gul'dan was torn apart by demons in the Tomb of Sargeras, but his skull kept a sliver of his malice. It passed through several orc hands until Grom Hellscream brought it to Ner'zhul, who used it to help reopen the Dark Portal. Khadgar later sent it to Dalaran together with the Book of Medivh, and the Kirin Tor guarded both.

As Dalaran fell in the Third War, the dreadlord Tichondrius stole the skull, and its power fouled the forests of Ashenvale and Felwood. Illidan Stormrage consumed its magic, became half demon, and slew Tichondrius with that power. The husk of the skull stayed with Illidan.