Your heart will explode.
C'thun is one of the five Old Gods imprisoned within Azeroth. In ancient times, C'thun and its kin ruled Azeroth, commanding vast armies of elementals, and raining chaos across the world at will. When the Titans arrived at Azeroth, they were horrified at the state of the world, and immediately set about trying to set right and orderly what clearly must have gone horribly, horribly wrong.
C'thun itself fought against a Titan in single combat, on the ground that would one day be Sithilus. It was defeated, presumed dead by the Titans, and the Titan C'thun was said to have fallen (whether that meant it died, succumbed to corruption, or tripped is deliberately vague). In the end, the Titans won their war against the Old Gods. C'thun and its kin were defeated and imprisoned within Azeroth, and their elemental minions banished to the Elemental Planes. The Titans arranged Azeroth to their liking, and created specialized defences to contain the Old Gods and their minions forevermore. Content in their reordering of Azeroth, and that their defences were strong enough, and possess sufficient redundancy to survive any possible contingency, the Titans left Azeroth in the capable hands of their chosen Prime Designate, the Watcher Loken, and were absolutely certain nothing could possibly go horrible, horribly wrong again.
Your friends will abandon you.
C'thun watched and waited as life grew and evolved on the surface of Azeroth. Finding the silithid to have potential, C'thun uplifted them, reforming them in his image, and granting them sentience. These uplifted silithid, the Aqir, would spread across Ancient Kalimdor like a blight, their empire spreading from modern day Silithus to modern day Northrend. The trolls of the Gurubashi and Amani Empires, however, didn't like their expansionist attitudes, or the non-troll competition in empire building, and went to war against them. The trolls defeated the Aqir, and broke their empire into at least three different pieces. C'thun remained in direct control over the Aqir who remained in Sithilus, further directing their evolution to suit its needs and desires. The Aqir of the south, now known as the Qiraji, continued to worship their Old God, giving it its name, and helping it to reform itself on the surface of Azeroth.
Biding its time, watching as the mortals went about breaking stuff, like the entire planet, C'thun watched its army grow from their fortress city of Ahn'Qiraj. When it was ready, he unleashed the Qiraji upon the world. Expecting to meet little resistance, the Qiraji advance was suddenly halted by the Night Elves led by
Fandrel Staghelm. The War of the Shifting Sands waged on for some time, but ended with the Qiraji trapped within their city. C'thun felt like it was imprisoned in a prison in a prison. It did not appreciate the redundancy the Night Elves and their dragon allies had unwittingly caused. Still, it wasn't a complete loss. Its Qiraji servants had learned some valuable lessons, and it now had several dragon they'd captured to experiment with. Biding its time once more, waited for someone, possibly an adventure looking for loot, title or cool mount, to reopen the Gates of Ahn'Qiraj.
You are already dead.
C'thun's patience was rewarded, and the Gates were reopened, with an army, the Might of Kalimdor, waiting outside. C'thun's Qiraji minions fought valiantly, but in the end they were defeated.
General Rajaxx,
Moam,
Ossirian the Unscarred,
The Prophet Skeram, all those royal silithid bugs, and even
The Emperors were all killed. Soon, C'thun found himself staring down like forty adventurers as they stormed into his inner sanctum in the Temple of Ahn'Qiraj. As they say, it's all fun and game until someone loses an eye, and C'thun had one really big eye to lose.
https://www.youtube.com/watch?v=tLCE18C6fKQ
You are weak.
A massive planetary parasite cannot be so easily slain, even by adventurers. C'thun was able to find an ally in the ogre mage and leader of the Twilight's Hammer,
Cho'gall. Cho'gall traveled to the ruins of Ahn'Qiraj and made an effort to restore its avatar. His attempt to summon C'thun was, however, stopped. However, Cho'gall body had been warped by the experience, taking on aspects of C'thun's form as a result of the failed summoning. The ogre mage continued to serve as C'thun's agent until his own death at the ends of adventurers.