Doomquest only appears once in the entire series, and that is as the leading villain in the episode, "Doomquest" which is the Season 9 finale. Doomquest is an intergalactic ruler from a dimensional world called the Dark Realm. Like Lord Dregg, Doomquest is a destined conqueror of the universe and will stop at nothing to achieve it.
For centuries, Doomquest has sought the Vortex Crystal, a powerful fragment that can warp space and destroy dimensional barriers. Doomquest plans to use the Vortex Crystal to fuse the universe with his own world. He tracked the crystal to Earth where he finds that Dregg had taken it. He sends his minion Draconos to capture the crystal, in which he eventually succeeds after a scuffle with both Dregg and the Turtles. Doomquest then uses the power of the Vortex Crystal to tear down the dimensional barriers between this universe and his, allowing the two worlds to fuse and become one. With a little help from Dregg however, the Turtles and Carter manage to take back the crystal without Doomquest's notice and use its power to reverse everything he's done and banish him and his army of dimensional monsters back to the Dark Realm.
Powers and abilities
Doomquest is one of the most powerful beings in the entire series, possessing an uimaginable level of strengh-enough to easily shake off and knock back two of the Turtles in their super mutated forms with one arm, both of which are strong enough to level and shatter a dam.
Doomquest can also fire very powerful lasers from his eyes and fingers/hands, and could charge matter which can be seen when he picks up a rock and energizes it. Doomquest has fought off and overpowered the Dreggnaught, which can destroy a planet as seen in the episode, "The Day the Earth Disappeared".