After completing Acerola's trial, you can enter 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. She asks if you want a Fresh Water or a normal water. It doesn't mater which one you choose. She gives you a Fresh Water and an Adrenaline Orb.
Go north to enter the desert. Go to the Haina Desert map
From the entrance of the desert, go east to the next area. Talk to the hiker. If you give him a Fresh Water, and if you found the Stufful in the motel in Route 14 earlier, the man will thank you and go back to the motel. If you visit him later, he will give you 10 Fresh Waters. Go east and get the Max Potion. Then go west to the previous screen.
Go north to the next screen. You can go north from here to find a Hiker. He will give you Safety Goggles. Then go south to the previous screen, and then go east to the next screen. In the middle, there is a small tree. Go north to the next screen, then go east to the next screen. Go east in this area to find the Psychium Z-Crystal.
From the screen with the Psychium Z-Crystal, go west. This area looks like the one you just came from, but it's actually a different area. 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. 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.