Root Kit?

It was the Plant stone. It worked much like the normal stone, except that any wishes made using it stood a 3 in 4 chance of inflicting a plant based partial transformation on both the person making the wish and any targets of the wish. The transformations had a built in limit of a maximum of 75% of the original person, so any wishes that produced an effect over that would undo previous ones to maintain the limit. The shadowy figure smiled to itself in the dark, and silently left the room.