Eye of Sargeras
The Eye of Sargeras is a gem-like artifact previously used by the dark titan Sargeras. It was apparently hidden there by Aegwynn when she defeated him, alongside his Avatar's body and the Jeweled Scepter of Sargeras. It was found in the Tomb of Sargeras by Illidan Stormrage. The artifact was sought by both Illidan and Gul'dan, the former gaining it only through the aid of the naga, the latter being torn apart by crazed demons when his entourage was defeated. The Eye of Sargeras was used by Illidan in an attempt to destroy the Frozen Throne, but the intervention of Maiev destroyed the Eye instead.
It is unknown if the Eye was Sargeras's actual eye, or simply an artifact in his possession when he was "killed" by Aegwynn.