Truman Marsh's parents were killed during a superhuman showdown between heroes and villains, blaming both sides for their deaths. This began his drive to stop super beings, so he became the warden of The Vault, a super-maximum security prison.
While working at the Vault he became a ruthless warden. There was a direct line from the White House to the Vault that would allow the President of the United States to detonate the facility if there was ever a breakout. Marsh, however, tapped into that line, allowing him the ability to detonate the prison on his own terms.
Marsh was also running illegal experiments on the inmates. An accident with low-level psychic Mentallo's increased powers which allowed for contact Venom in the next cell over, and the two work out an escape plan. Mentallo pretended to still be in pain from the experiment, and one of the Guardsmen came to investigate, forcing for Venom's cell to be opened before being knocked out with a psychic attack; It was during the commotion that Electro was freed and began realizing about the other inmates. Some of the inmates make it out of the Vault but were rounded up by the Avengers and Freedom Force. Venom began threatening Marsh to be released or Venom would begin killing the guards. Marsh started the timer on the bomb. But when he realized this was premature he tried to deactivate it but couldn't. He decided to destroy the wiring so it could not be stopped and then tried to leave via a secret elevator, but was attacked by Venom. Marsh, however, detonated the elevator and caused adamantium plates to seal the shaft.