100 years ago, Gregor was a beloved and powerful wizard.

While keeping up appearances in public and rising in the ranks as a trusted court magician, he hid his true nature as a manipulative user of forbidden magic with an insatiable desire for power. Practicing dark magic eventually began to corrupt his physical body, and once it became too advanced to continue in hiding he completed his transformation into a horrific beast.

The kingdom he belonged to and ones neighboring it were destroyed nearly completely by the monster, until the last few surviving wizards sacrificed themselves to capture and seal it in an impenetrable prison deep in the earth.

Impenetrable, that is, until a century later, when Gregor re-emerged.

A misstep was made while sealing him away, sealing his ability to use magic, but not his body's ability to produce it, and 100 years of built-up magic was too strong to be held by the seal.

He's just as surprised as everyone else is that he is alive again, and seems especially puzzled to be mostly-human again. He has no memory of his life after his corruption began, and after a century the accounts of his rampage as the monster have become too muddled with fiction and collective fears to discern the truth from. With his magic stripped, the only threat he really poses is physical. He's still a pretty terrible person, but seems more focused on piecing his past together and getting his magic back than actively being a menace, and mostly just toys with people nowadays.