After defeating Hau, you will become the first ever Alola Region Champion. In honor of your achievement, there will be another festival held in Iki Town. There to celebrate will be all the characters you have come across during your journey, including all of the Trial Captains and Kahunas, Professor Kukui, Hau, Lillie, and your Mother. While the festival is happening, a number fo scene will flash before your screen, detailing what has now happened in Alola.
The first scene will tell of Team Skull. After all that had happened with Necrozma, Guzma has decided to disband Team Skull, shown by the giant letter X he has placed on the logo on the back of his jacket. It appears as though his fellow members have deserted him, only for them to return without their Team Skull getup. They are still friends and loyal to their former leader.
The next scene will be of Zygarde waiting for you in Resolution Cave, followed by Dexio and Sina at the lab on Route 16. By going to Zygarde later on, you can battle it. You can then use said Zygarde - assuming you capture it - in the lab on Route 16 to eventually obtain a Zygarde with its Power Construct Ability.
The following scene will be Colress arriving at Aether Paradise. He is there to meet with the Ultra Recon Squad once more to tell them of Necrozma's capture, appearing as though they are still teaming up with their efforts on Ultra Wormholes. As they are having a conversation, Colress notices Faba mysteriously leaving Aether Paradise.
After that, the next scene will show Gladion purchasing a ticket to leave for Kanto and Johto. As he boards the ship, most of the Aether Foundation is there to see him off, including Wicke and a number of employees. As he continues to sail away, he notices Lusamine arrive and wave goodbye. He waves back with a smile.
The final scene shows the end of the festival, will all of you gather at Tapu Koko's Shrine. As all of you depart, Tapu Koko appears and waits to battle you. As your group heads backs to Iki Town. Lillie hangs back for a moment to look at the bridge where Nebby was first attacked by the group of Spearow, where she had the privilege of first meeting your player.
The credits will then roll, leading you into the events of the Post Game Storyline.