Once, Gregor was a talented and charismatic court magician. No one remembers him that way now. 

Wicked and power-hungry, a practitioner of evil magic behind closed doors, Gregor kept up respectable appearances until he became powerful enough to master forbidden form-changing magic. The Beast destroyed neighboring villages and nearly toppled the kingdom until he was stopped by the few surviving wizards

Unfortunately, the shackles had an unintended effect. They didn’t stop Gregor from producing magic, only rendered him unable to use it. As a sorcerer with innate powers, 100 years worth of magic with no outlet simply absorbed back into his physical body.

In other words, after 100 years... one day he just woke up and broke out of his prison with his bare hands.

He’s virtually unkillable now, since he’s full of 100 years of evil magic and can't be kept in prison long before he breaks back out again, so he's... kinda just left to hang around bothering the populace as he pleases? He’s still a terrible self-obsessed person who manipulates and makes the lives of others worse for fun wherever possible, but he’s generally pretty tame about it nowadays. more annoying and playful than an active threat unless provoked.

• After a century, no one who had seen him in person before his imprisonment is still alive, and accounts and stories about The Beast are often conflicting. The Beast existed, but its image in collective consciousness isn’t consistent. It’s difficult to say for sure which depiction of its form or recounting of its rampage is an accurate one. This bugs the hell out of Gregor, who honestly can’t remember much either-- he’d love to have more ammunition to torment people with.

• Used to be a prettyboy heart-throb before his heel turn. he’s still very vain but in a very opposite way? Now he gets his ego boosts from scaring people and being terrible.
• Refuses to do anything with his hair besides let it stay wild and tangled. dislikes having it touched.
• Despite having devoured many humans a century ago, he (luckily?) has no taste for them anymore. Nevermind that he tested this by biting off the last 3 fingers of a guard’s hand...