Little is known about Erron Black's background stories. From some cutscenes and dialogues, he is possibly a cowboy from Texas with excellent gunslinging skills and somehow managed to live over 100 years depending on Outworld magics. As an earthling, he lives in Outworld and serves Kotal Kahn.
Mortal Kombat X (semi-canon)
Years before the game, Erron Black had whacked a target under contract from Shang Tsung. As a reward, his aging was slowed down. In the present day, with his viper-like patience, he manages to take down Cassie Cage and her teammates for causing Kotal Kahn trouble.