Bring the the Husk of the Old God, 2 Idols of Rebirth, 5 Bronze Scarabs and 5 Ivory Scarabs to Vethsera inside Ahn'Qiraj. You must also attain Honored reputation with the Brood of Nozdormu to complete this quest.
When the stars are favorable, he will lash out against all living beings with terrible wrath. You must find him before then and find a way to bring his existence to an end.
Do this and bring me a piece of his husk. I shall grant you armor no mortal has ever worn... armor fit for one who has slain a god!
I did not expect you to return from your attempt, <name>.
You've fulfilled a destiny greater than that of most immortal beings.
By slaying C'Thun you've not only saved all of Azeroth, you've also tipped the scales of cosmic events beyond your comprehension.
Accept this breastplate as a symbol of the glory and burden that will come as a consequence of your actions.
May its power aid you in the challenges that await you, god-slayer!