After you complete Acerola's Ghost-type trial, you can explore Haina Desert.
Explore the Desert
Go to Route 13 and go north from the oasis to the entrance of Haina Desert. If you have completed the Ghost-type trial, talk to the girl to open the barricade. Then talk to her afterward and give her a Fresh Water to get three Adrenaline Orbs.
Go north to enter the desert. Go to the Haina Desert map
From the entrance of the desert, go east to the next area, where there is a Max Elixir. Then go west to the previous screen.
Go north to the next screen, then east to the next screen. In the middle, there is a small tree with a green Zygarde sparkle above it. Go north to the next screen, then go east to the next screen. Go east to find the Psychium Z-Crystal.
From the screen with the Psychium Z-Crystal, go west. Use Mudsdale to go over the rocks in the middle and get the Comet Shard at the center.
Go south from there to the next screen, where you can get TM85 Dream Eater.
Then go north until you reach the Ruins of Abundance. There is a green sparkle for the Zygarde Cube there. You can't do anything in the Ruins of Abundance yet, but it has been added as a place to which you can fly with Charizard.
To leave the desert, you can either use Charizard to fly out, or you can go south from the ruins, then west, then east, then north, then keep going south to return to Route 13.
From there, you can go west to reach Tapu Village, and go west from there to go to Route 15, where you should go back to Aether House.
Post-Game
After you become the Champion, you can get an item from a bald man in Haina Desert, at the location where you found the Zygarde Cell in the desert. For more information, read the post-game page.