Doc Benton discovered a formula that made him immortal. It had a catch though as body parts kept wearing out he had to replace them.
Doc Benton is a former man who is now a zombie-like serial killer. He needs to kill victims on a steady basis to replace his organs as his body decomposes. By doing so, he is able to continuously cheat death.
Doc Benton appears only in Supernatural: Origins:vol:18794 issue no. 4 where he is seen for the first time and killed by John Winchester who cuts him him in half at the waistline with a chainsaw.
This though didn't kill Doc Benton as he was able to put himself back together and then continue his spree. He was tracked down twenty years later by Sam and Dean. Sam wanted his Immortality formula to save Dean. Seeing the cost of the formula Dean refused and Doc Benton was buried alive with his Formula.