Arven asks you to investigate the Quaking Earth Titan in Asado Desert.
How to Find the Quaking Earth Titan
The Quaking Earth Titan can be found in the west part of Asado Desert, which is northwest of Cascarrafa.
Battle the Quaking Earth Titan
The Titan will be walking or rolling around in the desert. Bump into it or allow it to bump into you, and you will battle it. In Pokémon Scarlet, you will battle Great Tusk, and in Pokémon Violet, you will battle Iron Treads.
After you lower its HP, it will go southwest. Follow it and approach it, and there will be a cutscene. Afterward, you and Arven will have a double battle against the Pokémon. Arven has a level 44 Scovillain.
After you defeat it, there will be a cutscene, and you will get the Sour Herba Mystica. During the cutscene, you can choose to give your sandwich to Koraidon/Miraidon. Regardless of your choice, the Pokémon will gain the ability to glide. You can glide by pressing B to jump, then press B again while still in the air. If you are gliding, you can press B to stop gliding and start falling.