C'thun is one of the Old Gods from the Warcraft universe and the first to appear in World of Warcraft as the final boss of the "Temple of Ahn'Qiraj" raid dungeon from patch 1.9. He was one of the the two main antagonists (the other is Kel'Thuzad) in classic World Of Warcraft.
C'thun is the controlling force behind the Qiraji race, which he created based on the previously existing Silithid race of insects. When fought, C'thun initially appears as a giant eye, but when that it destroyed his true form is revealed as a bizarre tentacled mouth creature with enormous fangs. Players can also hear him "whisper" to them whenever they are in the Ahn'Qiraj dungeons.
An Old God that looks exactly like him also makes a cameo appearance in one of the Burning Crusade quests where a cult of Arakkoa are trying to summon it in Shadowmoon Valley. It is unknown whether it is C'thun or another Old God.