The most well known of the avatars is Thanos, his devotion to Lady Death having earned him his position. His rival as an avatar was the Magus, the darker personality (initially alternate timeline version) of the being known as Adam Warlock. In an effort to prevent Warlock from realizing his destiny, the pair would embark on a adventure that would prevent Adam from becoming the Magus and thus, the Avatar of Life.[1]

Phyla-Vell would also later become the Avatar of Death, having been appointed by the cosmic entity, Oblivion. In order to secure her lover Moondragon's freedom, she would become an avatar tasked with the goal of killing the avatar of life when he resurfaces, in which was revealed to be Adam Warlock when he re-asserted the future he aborted through magic during the War of Kings, melding himself and the former alternate Magus into one timeline and making the Magus a genuine evil counterpart to himself.[2][3]
Other entities have also been known to be Avatars of Death such as the Phoenix Force.

Other Earth-616 Avatars of Death
Alternate Realities