After you clear the massive mass outbreaks in Obsidian Fieldlands, you go to the Crimson Mirelands to investigate more.

The Request

After you complete Massive Mass Outbreak in the Fieldlands, approach Arezu and Mai in front of the hairdresser's in Jubilife Village. There will be a cutscene. You will get some Aguav Berries. The map will open. You can close it to proceed.

Investigate

Go to the Crimson Mirelands. Go to one of the massive mass outbreaks on the map. Sparkling locations are special outbreaks, and locations with a berry icon mean that when you defeat or catch pokémon there, you will get Aguav Berries, which you can feed to Mai's Munchlax to reveal massive mass outbreak details on the map again. Keep catching or defeating the pokémon at the location of the outbreak, and a message will appear when you have cleared all the pokémon away. They may be replaced with a new horde of evolved forms or alpha forms of the pokémon at the same location. If you take too long and the massive mass outbreak ends, go back to Jubilife Village, then go to any area outside of the village, and keep going back and forth until another massive mass outbreak occurs.

Report to Mai

After you have cleared away all the pokémon in three massive mass outbreaks, talk to Mai.

Reward

You will get 20 Apricorns, 30 Black Tumblestone, and 30 Iron Chunks.