Another piece of equipment was looted as Lu Li shut his eyes in pain.
Skull of Gul'dan!
This was the Skull of the Orc Warlock Gul'dan, which held a terrifying power inside that could draw out the evil in one's heart. After Illidan Stormrage got this treasure, he turned into a powerful demon.
It was said at the time, in order for Gul'dan to gain even more power, he went to Sargeras' tomb and died there in the end. His body was shredded into pieces by the demons in the grave, while his head was taken away by a Storm Raider Warlock back to Draenor.
This skull changed hands a few times in the battle.
Harkkan Skullsplinter, chief of the Ribchewer clan, ended up with it and hung it in front of his own chest, all the way until Grommash Hellscream paid him a visit.
Grommash killed Harkkan, subduing the Ribchewer clan and giving the Skull of Gul'dan over to Ner'zhul.