Skull of Gul'dan (Warcraft III)

The Skull of Gul'dan appears in A Destiny of Flame and Sorrow. Unlike other items, it appears as a building that can be targeted and destroyed in order for Illidan Stormrage to consume it.

Its 3D model was originally intended for a building called the "Altar of Chaos", which could have been the Altar of the planned Burning Legion faction.

Speculation
Rather than an oversized skull, the building may have represented the skull and its altar. The Cataclysm quest The Skull of Gul'dan (quest) implies that a red crystal was used as an altar for the Skull of Gul'dan.