Onyxia serves as one of the secondary antagonists in Classic World of Warcraft and one of the supporting antagonists in Cataclysm.
Onyxia speaks Common, Darnassian, Draconic, Dwarven, Orcish, Thalassian, and Zandali.
Long ago, Onyxia drove the Stonemaul ogres from the caves in Dragonmurk, not with any specific ill will, but merely for a place to raise her numerous children, who would then leave to infest the Wyrmbog. She and her children became known for terrorizing any travelers who strayed too close to her home.
Since Deathwing's departure, his eldest children have risen to lead their brethren and recoup their failing numbers. Deathwing's eldest son, the malicious and cunning Nefarian, has claimed the burning fortress of Blackrock Spire as his aerie. There, aided by his loyal dragonspawn, he subverted the remaining Blackrock Orcs and ensnared the black dragons not already under his control.
One of Nefarian's mightiest supporters is his younger sister, Onyxia. A clever, highly intelligent dragon, she delights in corrupting the mortal races by meddling in their political affairs. To this end, she takes on various humanoid forms and uses her charm and power to influence delicate matters between the different races. Onyxia has even assumed an alias once used by her father: the title of the royal House Prestor.
Though Onyxia is relatively small for a black dragon of her age, she shares the same powers and abilities as the rest of her terrible flight. Her searing breath weapon can melt both stone and iron. Her physical attacks, whether bites, scrapes or tail lashes, are lightning quick and nearly unstoppable. When cornered, Onyxia is fond of taking flight and blasting her troublesome prey from above.
Though Onyxia has been sighted traveling to and from Blackrock Mountain, her main lair is located in a fiery cave below the Dragonmurk, a dismal swamp located within Dustwallow Marsh. She is guarded there by her kin, the remaining members of the insidious black dragonflight. There also her brood of new eggs is hidden, awaiting maturation. Should anyone ever threaten Onyxia within her lair — within distance of her precious eggs — her wrath would be terrible beyond comprehension.
Onyxia aided her brother, Nefarian, with the orchestrations of their human personas, in order to work toward replenishing the black dragonflight. The heroes of the world have bravely faced these challenges, struggling to ensure the continued survival of their races.
For a time, Onyxia masqueraded as one of the highest officials in Stormwind, preventing the people there from sending much-needed aid to the areas near the Burning Steppes (where her brother has great power and influence). Her own lair lay in Dustwallow Marsh on Kalimdor, and she considered it a safe haven were her true identity to be discovered in Stormwind.
Ultimately, she was unmasked by Marshal Windsor with the help of Varian Wrynn, Broll Bearmantle, Valeera Sanguinar, and Thargas Anvilmar. Taking Anduin Wrynn captive, she fled back to the safety of her lair with Varian and his entourage in pursuit. Varian, Broll, Valeera, Thargas, and Jaina Proudmoore descended into the depths of her lair to rescue Anduin and bring her to justice. In the end she was slain by Varian, and her head taken back to Stormwind to be hung from the gates.