Arnie Gunderson was one Peter Parker's fourth grade classmates. At some point, Arnie developed mutant powers. Arnie gathered together several mutants called the Band of Baddies. With his team, Arnie, as Critical Mass, abducted a young mutant named Mary. Both Wolverine and Spider-Man went to rescue young Mary, whom Wolverine calls possibly the most powerful mutant. Critical Mass and his Band of Baddies forced Mary to aid them in repelling Spider-Man and Wolverine. She did, but the two quickly recovered and returned. One of the Band of Baddies threatened Mary at gunpoint and she unleashed her powers destroying the warehouse the villains were in, and supposedly killing them as well.
Critical Mass had the ability to project an explosive force from his fingertips. It is unknown if he kept is powers after M-day