Old God

The Old Gods are a race of naturally evil alien creatures from the Warcraft universe. They are a reference to the Cthulhu Mythos, but unlike the entities in the Cthulu Mythos, who have their own morals that are not understandable to mortals and cause harm unintentionally, the Old Gods are being of pure evil that intentionally cause harm and seek to bend the world to their liking.

History
Millions of years ago, the world of Azeroth was ruled over by the evil Old Gods and their elemental minions. Eventually, their rule was challenged by the peaceful Titans, who sought to bring peace and order to the universe. A long and bloody war was fought, but the Old Gods and their minions were defeated. Unfortunately, the Old Gods created a disease called the Curse of Flesh, which infected the Titans' iron minions, granting them flesh. The Titans knew that if they destroyed the Old Gods, their peaceful creations would be destroyed as well. Instead, the Titans chained the Old Gods beneath Azeroth's soil, and banished the Elemental Lords back to their own realms.

The Old Gods eventually found followers in the Twilight's Hammer Cult, led by the Ogre-Mage Cho'gall. The Twilight's Hammer seeks to release their chaotic masters so that they can unleash their evil and destroy the world. The Twilight's Hammer has been stopped countless times by the adventurers of Azeroth, and are even possibly in great confusion after Cho'gall's recent death. This will not stop the Old Gods' plans for world domination, however, and will stop at nothing to get their wish.

Named Old Gods
Although there are possibly countless Old Gods among the cosmos, only three have been confirmed to exist.

C'thun
C'thun was the first of the Old Gods to be named. He was heavily worshipped by Cho'gall himself, who was even greatly mutated by C'thun's influence. C'thun was the only Old God to not be imprisoned. Rather, he was believed to have been killed by the Titans, but has secretly been sleeping within the desert of Silithus for millions of years. He created the Qiraji, his mortal, insect-like avatars. The Qiraji are the dark masters of the Silithid, a less intelligent (though still quite intelligent) race of giant insects. During the War of the Shifting Sands, C'thun's legions warred against the Night Elves and Dragons, but was eventually defeated. The Night Elves and the Dragons built the Wall of the Shifting Sands to ensure that the Qiraji and their dark master do not spread their influence across Azeroth.

In time, C'thun began to reawaken, and the Second War of the Shifting Sands began. This time, adventurers were tasked to travel across the world in an effort to find the Scepter of the Shifting Sands so that they could assault the temple city of Ahn'qiraj. After months of searching, the scepter was reclaimed and the wall was brought down. The Temple of Ahn'qiraj was stormed, and C'thun was killed in a weakened state.

Not long after, while the Argent Dawn warred against Kel'Thuzad's undead legions in the Plaguelands, Cho'gall himself visited the ruins of Ahn'qiraj. With the kidnapped Orc, Med'an, Cho'gall attempted to use the Orc child's power to resurrect the Old God of Chaos. Med'an's friends came to the rescue, and defeated Cho'gall. Although C'thun was gone for good, Cho'gall survived and changed his loyalty to the evil dragon Deathwing.

C'thun's name is a possible reference to Cthulhu.

Yogg-Saron
Yogg-Saron was the second Old Gods to be named, and was imprisoned beneath the Titan city of Ulduar. Yogg-Saron's dark influence corrupted the Titan Watcher, Loken, and drove him to kill his sister-in-law, Siff. The heroes acted quickly, and killed Loken.

Not long after Loken's death, the heroes charged into Ulduar and battled Yogg-Saron's minions. They battled through his mechanical pets, his Twilight's Hammer Cultists, and even Yogg-Saron's top general, Vezax.

In a long and bloody battle, Yogg-Saron showed the heroes all that he had seen in his life. From what they saw, it can be assumed that he was what truly drove Garona Halforcan to kill King Llane. With the help of the other Titan Watchers of Ulduar, Yogg-Saron was finally killed.

Yogg-Saron's name is an obvious reference to Yog-Sothoth.

N'Zoth
N'Zoth is the third named Old God, but little is known about him. All that has been confirmed is that he was one of the three Old Gods to whisper to Neltharian the Earth Warder and cause him to go insane, changing his name to Deathwing, and leading a life of evil. It was also believed that he is the cause of the Emerald Nightmare, and is Xavius's true master. His two top commanders are Yor'sahj the Unsleeping and Warlord Zon'ozz, who were both killed during the Hour of Twilight.

Many clues have been given as to where N'Zoth resides. Using the Puzzlebox of Yogg-Saron, a player can listen to the voices of an Old God, presumably N'Zoth. Each whisper gives clues as to where N'Zoth is, but they are all mixed. It has been noted that he rests in a restless city called "Ny'alotha" that lies within black woods. It has also been theorized that Ny'alotha is at the bottom of the sea, as one of the whispers mentions this.

His name is a possible reference to Nyarlathotep and Azathoth.