Ula'ula Island Grand Trial

Walkthrough: As you meet Gladion on the ferry dock in Malie City, Nanu will appear and challenge you real quick to a battle. It turns out he's actually the Kahuna of Ula'ula Island, and this, no matter how rushed and unexpected, is your Grand Trial. He is a Master of Dark Type Pokemon, one with access to a Z Crystal.

Kahuna Nanu - Level 43 Sableye, Level 43 Krokorok, Level 44 Persian

After defeating him, you will receive your next Passport Stamp that will cause all Pokemon up to Level 65 to obey you. Kahuna Nanu will then give you the Darkinium Z-Crystal, directly before you, Hau and Gladion leave for your Return To Aether Paradise. But be warned: things this time around won't be as nice and pleasant as your previous visit.