Morghul was a young dragon hatched from the egg placed in the cradle of Princess Jaehaera, daughter of Aegon II and twin to Prince Jaehaerys, and bound to her, though she never rode him. The books give him no color; the illustration of the Dragonpit in The Rise of the Dragon paints him black.
When Rhaenyra's blacks took King's Landing in 130 AC, Jaehaera was spirited out of the city, but Morghul stayed chained in the Dragonpit with Dreamfyre, Tyraxes, and Shrykos through the queen's half year in the capital. When the Shepherd's mob stormed the pit he slew scores with his fire despite his shackles, until a huge man in heavy armor, remembered only as the Burning Knight, drove a spear through his eye even as the dragon bathed him in flame. Morghul died never having been ridden.