The Grim Reaper is a Marvel Comics character.  His real name is Eric Williams, and he is the brother of Avengers member Wonder Man.  Following his brother’s death and the use of his brother’s brain patterns for the mind of the android Avenger the Vision (long story, just go with it), Eric sought out the aid of the Maggia (Marvel’s version of the Mafia).  They gave him a techno-sythe, which had the ability to put people into comas, and he took on the persona of the Grim Reaper.  He attacked the Avengers, but was eventually defeated by new member Black Panther.  He dug up his brother’s dead body (eww, that’s gross) and took it to the Lethal Legion to have him brought to life.  Through a complicated series of events, Wonder Man returned to life and rejoined the Avengers, which angered Eric.  Eric became a recurring foe of the Avengers, died and came back a few times, and just took a generally evil stance on life.