Lord Mar-Vell is an evil, alternate universe version of the noble hero Captain Marvel.
Whereas Captain Marvel died of cancer, Lord Mar-Vell was saved by the primordial beings known as the Many-Angled Ones, who granted him immense power, which he later used to invade the primary Marvel comics universe.
In an alternate universe, when the noble Mar-Vell was dying of cancer, he was approached by the primordial beings known as the "Many-Angled Ones" who told him of a way to save himself; by destroying the manifestation of death in the universe. Mar-Vell agreed, and was granted immense new powers by the Many-Angled Ones, which he used to transmute many of his closest friends and allies, turning them into horrible perversions of their former selves.
Mar-Vell, now the Avatar of Life, sacrificed the Avatar of Death in his universe in a ritual known as the Necropsy, which destroyed Death and allowed the Many-Angled Ones to flow freely in the universe, leading to the corurption of all organic life.
Due to the actions of the Inhuman King Black Bolt of another universe, a tear in the multiverse was created, creating a gateway between Mar-Vell's universe (known as the Cancerverse) and the primary Marvel Universe, Earth-616. Mar-Vell sought to do to this universe what he had done to his own, and began searching for Earth-616's Avatar of Death, Thanos the Mad Titan. Mar-Vell finds that Thanos, along with the Guardians of the Galaxy, had been moving about in his own home universe, and quickly left in search of them. Upon finding them, Thanos immediatelly surrendered and told Mar-Vell to grant him oblivion by killing him. Too late however, does Mar-Vell realise that he has been decived. By "killing" Thanos, he allowed Death to once again enter the Cancerverse, and she proceeded to kill every living being in it, including Mar-Vell.