The Skull of Gul'dan

After Gul'dan died, his skull was turned into a channeling totem for demonic energy. The skull was used by Ner'zhul to open portals to other worlds on Draenor, and later by Khadgar to destroy the Dark Portal. After the destruction of the Dark Portal, Khadgar left the skull behind on Draenor in his haste to escape through one of Ner'zhul's rifts. Years later it would appear again on Azeroth, this time used by the Burning Legion to corrupt the forests of Felwood. After learning of its existence from Arthas the death knight, Illidan Stormrage found and took the skull, absorbing its power, becoming half night elf and half demon.
[edit] Gul'dan in World of Warcraft
Vision of Gul'dan
Vision of Gul'dan

A vision of Gul'dan can be seen at the Altar of Damnation of Shadowmoon Valley in what is now Outland - the remnants of Draenor. The altar lies in front of a massive volcano known as the Hand of Gul'dan.

The repeating vision appears to depict an event prior to the opening of the Dark Portal, where according to the Earthen Ring, the volcano was raised from the valley floor by Gul'dan himself when he severed the orcish people's connection to the elemental spirits of Draenor.