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Colossi are giant lumbering creatures that appear in Outland. They are said to be "mountain giants". Their flesh is stony, and giant crystals stud their shoulders, back and skull. If they are killed, they can be mined after they've been looted.

On Hellfire Peninsula, after a red crystal fell to the surface from the sky, artificial colossi, led by King Goliathon, appeared. The red crystal that summons them and drives them mad was created by Pathaleon the Calculator for his master, Kael'thas.

There are colossi found in other places that are natural to Outland. These true colossi are led by Behemothon, King of the Colossi in Shadowmoon Valley, and the Illidari are negotiating an alliance with them. Some of these natural colossi can be found in Netherstorm, and other locations of the world.

If Goliathon's colossi are natural, then Behemothon would be his sucessor when he is killed in the quest chain.

A named colossus, Tavarok, resides in the Mana-Tombs of Auchindoun.