Multitype changes an Arceus holding this item to its Poison-type form. From Generation V onward, item-manipulating effects (such as Trick and Fling) cannot give Arceus a Plate or remove a Plate from Arceus. (In Generation IV, item-manipulating effects cannot affect Pokémon with Multitype.)
The Toxic Plate, alongside the other 15 Plates at the time, appeared in Arceus and the Jewel of Life, where Arceus used them to change its type, and almost died when it lost all of the Plates after having destroyed a meteor that endangered the future land of Michina Town. Damos found one of these lost Plates and returned it to Arceus, giving it the strength to recall the other 15 Plates.