The Darkinium Z (Japanese: アクZEvil Z) is a type of Z-Crystal introduced in Pokémon Sun and Moon. It is the type-specific Z-Crystal associated with the Dark type and allows Pokémon to upgrade their Dark-type moves to Z-Moves.
Unaffected by item negating effects (such as Embargo and Klutz). Item manipulating effects (such as Trick and Fling) cannot remove a Z-Crystal from a Pokémon.
Nanu used a Darkinium Z in Guiding an Awakening! to have his AlolanPersian perform Black Hole Eclipse during his grand trial battle against Ash. After Ash defeated Nanu, Nanu contemplated giving Ash a Darkinium Z as a prize, but then quickly decided to give him a Lycanium Z instead.
A full set of type-specific Z-Crystals, including the Darkinium Z, appeared in SM109, where Hapu allowed Ash to choose his grand trial prize from amongst them.
A full set of replicas of the type-specific Z-Crystals, including the Darkinium Z, also appeared in A Team-on-Team Tussle!, where they were shown to Ash and his classmates by Professor Kukui.