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.
Petrel was collecting the Plates for Team Rocket's plan. After Silver stole the few Plates he had managed to find, Petrel decided to let him gather all 16 Plates, just to get them back at once later. With his Weavile leaving messages for Sneasel and Weavile across Johto, Silver soon indeed gathered all 16 Plates for himself. At the Sinjoh Ruins, Gold surrendered the Plates to Archer, who used them to seemingly take control of Arceus, ordering it to recreate Dialga, Palkia, and Giratina. When the Pokédex holders used their Pokémon's ultimate attacks to stop the creation, all the Plates were absorbed by Arceus.
The Toxic Plate was one of the two Plates that Petrel had found and were then stolen by Silver.