C'thun is one of the Old Gods from the Warcraft universe and is the Old God of Chaos. He is one of the main antagonists in the classic World Of Warcraft. He was the first to appear in World of Warcraft as the final boss of the "Temple of Ahn'Qiraj" raid dungeon from patch 1.9.
C'thun, like the rest of his horrific kin, is an interdimensional parasite of unfathomable power and evil. Eons ago, when Azeroth was still young, he and the other Old Gods ruled the entire world through the Black Empire.
The Old Gods are parasitic eldritch horrors that come from The Void. They are sent out into the cosmos by the Voidlords to seek and corrupt planets that have a World Soul in them. Should a World Soul ever fall victim to this corruption, it would give rise to a being of darkness that is so powerful it could bring about the end of all creation. This dreadful entity's power is so feared, that upon witnessing the possibility of one, Sargeras went insane, and would eventually form The Burning Legion in a hellish crusade to stop this from ever happening.
C'thun, Yogg-Saron, N'zoth, and Y'shaarj made their way to the newly forming Azeroth and slammed into the surface of the planet, then proceeded to root themselves in to begin spreading their madness and evil. These four primordial beings of corruption would give rise to their empire, The Black Empire, and rule ancient Azeroth, endlessly warring with one another to further bring ruin to the world.
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.