After you defeat the alpha pokémon in Deertrack Heights, Commander Kamado summons you the next day.

Talk to the Commander

Go outside and there will be a cutscene where you meet Adaman, leader of the Diamond Clan, and Irida, leader of the Pearl Clan.

Go into Galaxy Hall and go up the stairs until you reach Commander Kamado. There will be a cutscene. This completes the mission.

The next mission is The Frenzy of the Lord of the Woods.