After passing through the Ancient Poni Path and walking along Poni Breaker Coast, you reach the Ruins of Hope.
Ruins of Hope
Follow the path eastward. You will find a green sparkle southeast of the door.
Approach the door, then after the cut scene, go through the door.
Go north to see a cut scene.
You have to push the stones to open the path to the north. If you make a mistake, go south to leave the area, then go north to go back in. The stones will be back in their original positions. If you just want the answer, click the button.
After you solve the puzzle, go north to the next room, then go north to start a cut scene. After that, you can go outside and fly to Seafolk Village or go there the long way. Another cut scene will begin when you arrive.
Afterward, go to the southwest end of the village, and there will be another cut scene.
When you arrive on the island, approach Lillie and there will be another cut scene. You will battle a level 40 Exeggutor. You can catch it or knock it out.
After that, go north to talk to Lillie.
After the cut scene, check in the cave where you sheltered from the rain. There is a hidden Big Pearl there.
Go north, and go up along the eastern side of the island to get a Prism Scale.
Then go south, and go up the ramp to the left. Go north and check on the red flowers to the north to find a hidden Heart Scale.
Then go southwest and follow the path to the northwest end of the island. Talk to Lillie. Then take the flute from the pedestal.
Afterward, go to Ancient Poni Path, and go north to Lillie. You have to battle a Team Skull Grunt who has a level 44 Fomantis. You get 1408 Pokédollars for winning.
After that, you battle a Team Skull Grunt who has a level 43 Salandit, a level 43 Golbat, a level 44 Alolan Raticate, a level 43 Mareanie, and a level 44 Haunter. You get 1408 Pokédollars for winning.
Afterward, you get Poisonium Z, and Lillie heals your pokémon.
Go north to reach Vast Poni Canyon.