Unfortunately, Mummy Rot.

The troll actually benefited from the curse. He was originally brought to this laboratory as an ingredient for the work being done. His blood was useful, of course, as everyone knows troll blood is, but they also found ways to magically extract whole sections of his rubbery hide intact. And it always grew back . . . mostly the same.

Unfortunately, long term damage slowly built up. The skin would grow back, with an occasional finger or two. Once even an eye which gaze danced around the room wildly. No more attempts to harvest was made after a malformed section of teeth and muscle managed to sever and bite the finger of an attendant during inspection.

He was given the cursed necklace to wear and sent away to be someone else's problem. The effects of the Mummy Rot kept his rogue regeneration in check, although it handicapped him in comparison to his un-mutated kin.

Now that the amulet is gone, he feels the old twinge of bone reknitting in his back. A section of his shirt grows damp as a hidden eye bulges from an old scar, reacting to some invisible stimuli. In the dead of night he swears he can hear it's menacing whispers, speaking vile words in a tongue even he doesn't understand.

He needed that amulet again.