Omen is a special boss from World of Warcraft that only appears during the Lunar Festival - making him a seasonal enemy that can only be faced by players during a certain time of the year, he is one of several such bosses known as Event Bosses.
History
The demigod Omen was a being of great strength and knowledge. Blessed by Elune, Omen wandered primordial Kalimdor, guiding hunters and gatherers and offering luck and advice to all he met. When the Burning Legion launched their attack, Omen allied with Azeroth's defenders. Omen fought bravely, but after many battles, his wounds from demonic forces plunged him into a deep, dreaming sleep...where fel-inspired nightmares slowly consumed him. When he finally awoke, centuries later, he had fallen to madness.
The transformed Omen rampaged. No longer a creature of wisdom — but still possessing the strength of Elune's blessing — he tore through the land, killing and devouring. Terror rode before him, and death trailed in his wake.
Omen's spree of horror ended when a coterie of ancient heroes gathered in the village of Nighthaven and faced the beast. They called down the moon's wrath and Omen, blinded and stunned by the power of the moon goddess, fled into the deep waters of Lake Elune'ara. That day marked the end of his rampage, and symbolized a new beginning for the people of Azeroth, and the followers of Elune.
Omen, defeated so many centuries ago, yet remains in Lake Elune'ara. Although driven insane by the Burning Legion, the demigod still possesses Elune's blessing and is, hence, immortal. During the Lunar Festival, he stirs.. and heroes of the land are called to defeat him once again.