arena battle royal

story: you're 35 years old, and when you were thirteen you got bitten by a radioacive spider, but you couldn't control your power, so you trained saitama style (fyi: 500 pushups, 500 sit ups and a 100k run every day) until you were 34, then a couple month after your birthday, you were walking along the side walk and a portal opens up underneath you and you fall into an arena waiting room