He was around before that, much longer. Don't Starve was when he became The Agent, before he was just Maxwell, a servant of The Virus. He abosrbed the Virus when he least expected it: While the Avatars were just about to fight him. The Virus was a, well, a virus. He dragged a puppeteer named Maxwell Sanders and gave him the form of Maxwell from Don't Starve. Maxwell gathered crucible pieces from worlds for him because the multiverse saw him as a threat and put him into a firewall. As he got more powerful, the closer he got to escaping the protection.