Trolls and Trolls

I am still struggling with the whole trolls as character (like in World of Warcraft) versus trolls as monsters (like in almost everyone else).

I need to come up with some clever contrivance where I have both, simultaneously at once (yes, I already know, Department of Redundancy Department).

At some undefined time in the far distant past, a powerful sorceress transmogrified some monster trolls into character trolls, and there have been two distinct populations ever since.

Why does it always have to come down to the intervention of a powerful sorceress?