When he was alive, he was the mightiest of His Divine Shadow's Bio-Viziers, until His Shadow exiled him and he arrived on a barren snow-covered world. His physical appearance was that of a head situated on top of a large hovering jar (for lack of a better word), containing all of his organs. For limbs, he had two drone arms.
(Spoilers ahead)
When the crew of the Lexx paid a visit to Mantrid's world in search of protoblood, Kai, possessed by His Shadow's essence, brought along a dormant insect with them. The insect must be fully alive to extract protoblood and Kai gives them the location of Manrid, somebody who can help them. As they begin to wake the insect up, the essence of His Shadow travels from Kai to the insect, reviving it. Now free, Kai kills Mantrid but His Divine Shadow then transfers his essence into Mantrid from the insect via an organ from the insect. When the organ is destroyed, his essence, mixed with His Shadow's, manages to fuse with his ship's computer.
Mantrid escapes the destruction of his planet with his bizarre little ship, taking along his two drone arms. These arms can make more of themselves by disassembling raw material and building more, so his ship is followed by a host of dozens of these arms. He follows the Lexx, eating most of the planets they go to and making more drones. Each planet or station destroyed brings millions more drones in production. Soon the number of drones is enough to start dismantling the entire Light Zone, which Mantrid begins to do with haste. Soon he is taking apart stars to build more drones and the more there are, the more damage he can do.
He is eventually defeated by the crew of the Lexx at the season finale of Season Two. His supposed death is synonymous with the destruction of an entire universe and all of the trillions upon trillions of drones, giving the Lexx a very narrow escape into the Dark Zone. he manages to get onboard the Lexx as a single arm drone in a last attempt to kill them. When Kai destroys the arm, all that is left of him is the computer holding his essence, which Xev effectively smashes, putting an end to him as he screams "I destroyed a universe!"
Mantrid makes a short apppearence in the Third Season as well.