Mandrakk was originally Dax Novu, first "son" of the original Monitor. He was the first to map the multiverse and felt contaminated by it. Corrupted on his voyages, he eventually took the name Mandrakk and adopted a vampiric appearance. He was imprisoned (possibly by his own volition) for attempting to consume the Bleed (life in its purest form).
After his son, Nix Uotan was banished, Mandrakk was freed partially through the machinations of a lesser Monitor named Rox Ogama. However, he was defeated by Superman who was in control of the weapon Dax Novu created to protect the multiverse, the Thought Robot with vast cosmic powers. Mandrakk seemed to fall to his death into the Overvoid, although Rox Ogama was seen in the end plotting his return.
Mandrakk survives, beginning to make his move against Earth, using Darkseid's own attack to prepare the way. The Radiant, God's Mercy, and the Spectre, God's Wrath, both face him, attempting to stop him. Mandrakk is able to defeat both of them, feeding off of their power to add to his own. He then waited out Darkseid's attack for the right moment to strike.
Mandrakk later resurfaces after the demise of Darkseid, taking advantage of the destruction he's caused to finish dragging New Earth into the darkness where he lives. He attempts to gain revenge on Superman, using his controlled slave Ultraman, he is able to overpower and subdue his cousin Kara Zor-El. Showing that he has defeated both the Spectre, and the Radiant, he tells Superman to surrender, stating that victory is inevitably his.
He approached Superman, mocking him that his machine had failed him. Yet Kal-El surprised him, using the last of the light locked within his cells to bring the weapon to life. He then explained to Mandrakk that he had failed. Mandrakk was surprised by the arrival of the Supermen of the Multiverse, who wasted no time putting down Ultraman before turning their attention to him. Just at that moment, Nix Uotan arrived, claiming that this would be settled by the Monitors.
Nix then set about stopping his father. He began to activate the last ditch defenses of the Multiverse, fulfilling his duties as the Judge of All Evil. Mandrakk didn't recognize him at first as he used his power, calling Superman to his right hand, as well as creating Captain Carrot and his allies, teleporting the Green Lantern Corps there, the Angels of the Pax Dei, as well as the Forever People of the 5th world. Just then, as the Green Lantern's arrived, Mandrakk recognized his son. Nix shrugged it off, telling Mandrakk to return to the darkness he called home if he wished to survive.
He attempted to fight back but was stopped by the combined power of the Supermen of the Multiverse, Nix Uotan, angels of Pax Dei, the Forever People, and several Green Lanterns. The Supermen blasted him with their combined heat vision, but were unable to finish him off, though they did hurt him. Mandrakk is taken by surprise, confused that things weren't going as they had in the book of Limbo as he had read. He is finally killed when the Lanterns drive a giant stake through him. Though his true Legacy, as a Monitor and the first "son" of the Original Monitor was carried on by his own son, Nix, the very Monitor who had defeated him.
Powers and Abilities
Mandrakk in Final Crisis was a cosmic vampire. Immensely powerful, he was strong enough to defeat The Spectre and The Radiant together, and was able to barely survive being outside DC creation. He was shown to have the combined power of 52 universes (the multiverse in final crisis), and was bigger than the multiverse.
He could apparently "eat stories" for some reason, supposedly making him more powerful than most comic characters. The only story he could not change or eat was the origin story of superman.