Forest's Curse adds the Grass type to the target, in addition to the Pokémon's original type(s). If it is used against a dual-type Pokémon, that Pokémon will have three types at once. If the target already has an additional type added to it by Trick-or-Treat, that type is replaced with the Grass type. Forest's Curse will fail if the target is already Grass-type.
Phantump's eyes shine, and a ring of the same color shoots out of their eyes and hits the ground. The ground grows purple and purple vines grow out from it to constrict the opponent, turning the opponent into a Grass type.