After you get the seventh Gym Badge and investigate the strange red light, you are ready to challenge Raihan in the Hammerlocke Stadium.

Hammerlocke Stadium Gym Mission

When you are ready, talk to the person at the north end of the gym lobby to begin the Gym Mission.

First, you have a double battle with Gym Trainer Sebastian, who has a level 45 Sliggoo and a level 45 Pelipper. You get 4320 Pokédollars for winning.

Afterward, talk to the next trainer and you will have a double battle with Gym Trainer Camilla, who has a level 45 Turtonator and a level 45 Ninetales. You get 4320 Pokédollars for winning.

Afterward, talk to the next trainer and you will have a double battle with Gym Trainer Aria, who has a level 45 Hakamo-o and a level 45 Abomasnow. You get 4320 Pokédollars for winning.

After that, go north to battle Gym Leader Raihan.

Gym Leader Raihan

You will have a double battle with Gym Leader Raihan. He has a level 47 Flygon, a level 46 Gigalith, a level 46 Sandaconda, and a level 48 Duraludon, which he will Dynamax right away. You get 7680 Pokédollars for winning.

You get the Dragon Badge, which allows you to catch pokémon of any level. You also get TM99 Breaking Swipe, and a Dragon Uniform.

At Poké Marts, you can now purchase Full Restores.

When you go outside, there will be a cutscene.

Go east to Hammerlocke Station and go inside, then go east toward the turnstiles, and there will be a cutscene. Afterward, you arrive in White Hill Station. Go outside and there will be a cutscene, and you will arrive in Route 10.