Morghul was a young dragon bound to Princess Jaehaera Targaryen before the Dance of the Dragons. Morghul died in the Storming of the Dragonpit when it was invaded by a half-crazed angry mob after a one-handed prophet called the Shepherd began to rant against all dragons everywhere. There is little doubt that Morghul killed scores of people before being slain. Morghul was never ridden.
Shackled by heavy chains Morghul, it is written, was slain by the Burning Knight who rushed headlong into the dragon's flame with spear in hand, thrusting its point repeatedly even as dragonflame melted the steel plate that encased him and devoured his flesh within.