Gregorovic was not always a literal monster. He was a man that wished to discover the secret of immortality (as he was suffering from a terminal illness), so he decided to abduct some elves from the Cywir Clan and experiment on them, resulting in most of them dying.
This earned him the hatred of Helwyr, the leader of the Cywir Clan, who would make it his mission to hunt him down for what he had done.
As Gregorovic's experiments did not lead to him achieving immortality, he decided to change his plans. He made a deal with Sliske, agreeing to become his puppet in exchange for eternal life.
Sliske then turned him into a monster, because as he put it, he already was a monster, and the new look would suit him.
Gregorovic now had the ability to consume other beings in order to keep himself immortal. Eating powerful beings would cause him to become more powerful. He agreed to become Sliske's servant, eventually becoming his general.
Sliske would then send Gregorovic to the Heart of Gielinor in order to obtain the pool of anima there. In order to do so, he would have to kill Helwyr, Vindicta and Gorvek, and the Twin Furies, whose sister he had consumed.
He appears in Sliske's Endgame as one of the bosses.