Causes Feebas to evolve into Milotic when traded while holding it. This consumes the Prism Scale.
This item was introduced in Generation V due to the lack of a means to increase Beauty in these games. However, even in games in which the Prism Scale exists, a Feebas can still evolve if leveled up while its Beauty is high enough.
A Prism Scale appears during Chapter 4 of Detective Pikachu, being worn by Milo Green on his headband. He mentions that he obtained the scale when he was once saved from drowning by a Pokémon, which is later revealed to have been a Milotic. The presence of this Prism Scale also acts as the trigger to the evolution of a Feebas that Milo has befriended.