A key in remarkably good shape for it's age, it pre-dates the majority of known history.
Unlocks the door to the Ancient Ruins located somewhere within the Desert.
The Ancient Key is the reward for both guild questlines - the Assassins Guild in Darkhaven and the Guild of the Golden Circle in Fira. It is used to open the huge gates allowing entrance into Sandmans Dungeon.
6.3X+: In the Ice Temple where the portal to good diablo(EMQ) used to be there is a Chest (or Lock Box, can't remeber) that contains this key.