Cyrus Gold was a man in Gotham during the 19th century. Cyrus got into trouble with Gotham's Organized Crime world and found himself getting killed and dumped in the a wetland known as the Slaughter Swamp.
Within the Slaughter Swamp Cyrus's body bonded with the plant-life and he eventually emerged as an undead behemoth with little memory of his past-life. Cyrus assumed the new name of Solomon Grundy after a character in a nursery rhyme and became a ravenous monster. By modern day, Solomon came to live within the sewers of Gotham City and would eventually come to be a rogue of Batman along with several other heroes.