The Z-Power Ring is an all new item introduced in Generation VII. Like Z-Ring and alongside a Z-Crystal, Z-Power Ring can also trigger a Z-Move.
The Z-Power Ring is completely black everywhere except for face where it has white highlights and a grey shade where the Z ring should go.
The player character received a mysterious stone after being saved by Tapu Koko. After the battle with Hau, Hala gave the player character the Z-Power Ring, which was made from that mysterious stone.
Ash Ketchum, Gladion and Team Rocket have Z-Power Rings.
Z-Power Ring in the Anime
Team Rocket's Z-Power Ring