After you go up the mountain, you reach the summit, where you find the Crystal Pool.
How to Reach the Crystal Pool
One way to reach the Crystal Pool is to go through Infernal Pass.
Explore the Crystal Pool
Check on the small sign near the bridge to add this location as a Fly destination on your map.
West of the Crystal Pool there is a Terastal Glimmora.
Near the Terastal Glimmora, you can find a TM165 Flare Blitz.
South and slightly west of the pond, near the mountain wall, you can find a TM101 Power Gem.
Southeast of the pond, near the pond's edge, you can find a TM098 Skill Swap.
Also southeast of the pond, there is a cave entrance that faces the mountain wall. Make sure to check on the sign at the north edge of the Crystal Pool before you go into this cave, to add it as a Fly destination. If you go into the cave and fall down below, you can battle Kotetsu of the Kitakami Ogre Clan, who has a level 75 Heracross, a level 75 Gallade, a level 76 Conkeldurr, and a level 77 Kommo-o. You get 7084 Pokédollars for winning.
East of Kotetsu, you can find a TM116 Stealth Rock.
Southeast of Kotetsu, there is a tunnel. If you fall down the hole at the end of this tunnel, you will find a TM142 Hydro Pump.
The southwest tunnel from here just leads to a hole that goes to an exit to Oni's Maw, so go you can just back to the Crystal Pool now.
Back at Crystal Pool
If you went down the caves, fly back to the Crystal Pool. Go to the destination on the map and talk to Carmine. There will be a cutscene.
You will have a double battle with a level 73 Milotic. You can't catch it, so just battle it. Carmine sends out her Morpeko.
Afterward, you get the Crystal Cluster.
Now fly back to the Community Center in Mossui Town. Go to Carmine's house and talk to her grandfather. There will be a cutscene.
Now go to Loyalty Plaza (west of Mossui Town) and go to the quest destination. Talk to Kieran and there will be a cutscene. You have to battle Kieran.
Kieran has a level 68 Yanmega, a level 71 Poliwrath, a level 66 Gligar, a level 69 Dipplin, and a level 65 Cramorant. You get 4260 Pokédollars for winning.
Afterward, there will be a long cutscene. After that, go back to town to heal up, then go to the Ogre Oustin' location at Kitakami Hall.
Approach the Caretaker at the destination marked on the map and there will be a cutscene.
Next, you can fly to the Infernal Pass Fly destination on the map if you have it, or if not, go west to the road sign that says to go left for the Infernal Pass and go right for the Dreaded Den. Follow the sign to the Dreaded Den. Approach the pokémon outside of the Dreaded Den and there will be a cutscene.
You will now battle the level 70 Munkidori. You can't catch it right now, so just focus on defeating it.
Afterward, Ogerpon will follow you. You can either go to the destination on foot, or just Fly to the Community Center in Mossui Town, and there will be a cutscene. Next, go into town. Talk to the kid just east of the Community Center to learn about Fezandipiti. Go north of the Community Center and talk to the woman to learn about Okidogi. Go toward the west exit of town and talk to the man there to learn about Munkidori.
After you have talked to all three people, there will be a cutscene. Carmine will add the locations of Okidogi, Munkidori, and Fezandipiti to your map. When you use the map, zoom all the way out to see their locations. You can go to the pokémons' locations in any order. Read more at the Loyal Three and Ogerpon.