That's Alaktor, and that's his Marvel Belt, failing dismally. Alaktor later claimed to be a super-villain who was just using the Legion tryouts as a means to sneak into the Legion clubhouse and steal a time machine, but I think that that's just an excuse. He was totally just covering for his embarrassment over the tryouts - there's no way that he meant to press the button that made his belt go 'ROAR.' Dude screwed up and then improvised some evil to cover for it.
Sure, Alaktor. Sure. Master of worlds. Admit it - that camera was there for one reason: to take cheesecake pictures of the Legion ladies.

As evidence of Alaktor's inherent lameness I offer a quick rundown of his 'evil' plot:

He planned grab three of the most evil characters in history (Hitler, Nero, Dillinger) and transplant their minds into the most powerful of the Legionnaires. I have three problems with this plan:

1) Choice of allies. Hitler, Nero and Dillinger? Jeez, guy. These are the best you could come up with? Hitler, sure. He was straight-up evil, but really, did you expect him to be a staunch and loyal ally? Same goes for Nero - most of his evil consisted of doing horrible things to those closest to him. I can think of about a dozen Roman emperors who'd've been a better subject for cramming into Mon-el's brain, evil assistant-wise. Dillinger? Sure he robbed a lot of places and shot a lot of people, and sure he was reputed to be fantastically well-endowed (not inherently evil but I'm sure it could be turned to evil ends) but definitely not in the top three evil people of all time. Basically any other three people would have been better allies than these guys, who horribly betrayed him in about three seconds flat:

Dude, you totally should have known this would happen.

2) If you're putting someone's brain into Superboy... why not your own? Seriously, why do Hitler favours? Just take over the Kryptonian and leg it. You're set!

3) Dude, you have a machine that can switch peoples' minds. I'm pretty sure that that's worth something. There's really no need to give Nero superpowers. Just open an amusement park or a novelty bordello or sell it to an unscrupulous billionaire who wants to live forever - you'll be astonishingly rich! You'll be able to afford pants!