Walkthrough: As you meet Gladion on the ferry dock in Malie City, Nanu will appear and challenge you real quick to a Battle. Turns out he's actually the Kahuna of Ula'ula Island, and this is your Grand Trial. He is a Master of Dark Type Pokemon.
Kahuna Nanu - Level 38 Sableye, Level 38 Krokorok, Level 39 Alolan Persian w/Darkinium Z.
After defeating him, you will receive your next Passport Stamp that will cause all Pokemon up to Level 65 to obey you. Nanu will then give you the Darkinium Z-Crystal, directly before you, Hau and Gladion leave for your Return To Aether Paradise. But this Time around, things won't be as nice and pleasant as before.