Decay was once but an average worker in the Facility. One day he was called onto an investigation of a strange sample. This sample contained an even stranger object, a skull. Excavation crews refused to enter, so Decay was sent in to take it himself. Suddenly controlled, Decay lost perception as he was forced to touch the skull. An undetermined amount of time later, Decay finally regained consciousness, only to find the skull affixed to his head, and his actions not his own. Decay lay waste to the Facility killing anyone he found. Eventually, he was lead out, alive but remaining in agony, fully aware of his situation. Now he roams, constantly beckoned by the powers of the skull which he cannot escape.
Decay appears in The Dimensional Discrepancy 2, after being summoned by Master Hand in order to help kill bedrockperson, but ends up becoming the main antagonist after killing the Hand and proceeding to destroy the universe. When Bedrock finally faces him, he is swiftly defeated, but before finishing the job, the host fought back and plunged himself into lava. The victory was short lived, as Decay survives and becomes a Wither, but is defeated. Lawrence, and by extension Decay, dies by violently exploding, but the skull itself evidently survives and destroys the dimension. As the entire universe rebooted, it's unknown if Decay's skull regenerated as well.