I doubt there is no one left here that hasn't heard of Sargeras stabbing Azeroth on the end of Antorus.
So, here is a question. Considering the fact that C'thun dwells below Southern Kalimdor, and Southern Kalimdor being the place Sargeras stabbed with his blade, could C'thun be truly dead at this point? Like that one time Aman'thul ripped Y'shaarj to bits with his hands?
Personal opinion: I think C'thun is dead, because that stab was awesome, and it would be a epic fail if Sargeras missed it because he forgot to level his weapon skills.