Arven asks you to investigate the Lurking Steel Titan in East Province (Area Three).
How to Find the Lurking Steel Titan
If you go northeast from Levincia, you will reach East Province (Area Three). Continue going northeast on the path until it curves to the west. Go west along the path, then when it turns south, go south. Eventually, the path starts going toward the west. Keep following this path, and keep an eye on your map to see the location of the Lurking Steel Titan so you can go north to it when you are close.
Battle the Lurking Steel Titan
As you approach the destination on the map, you see a large Pokémon sticking up out of the ground. Approach it and it will run away. You will need to dash into it with Koraidon/Miraidon to chase it and run into it.
After you reach it, you will battle Orthworm. You can't catch it, so just focus on defeating it. It is immune to Ground-type attacks because of its Earth Eater ability, but it is still weak to Fire and Fighting attacks.
After you lower its HP, it will burrow into the wall. Enter the cave that it created, and go all the way through it to find the Titan again. Dash into it like before, and follow it. It will open another cave, and there will be a cutscene. Afterward, you and Arven will have a double battle against the Orthworm. Arven has a level 28 Toedscool.
After you defeat it, there will be a cutscene, and you will get the Salty 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 jump higher if you press and hold the B button. You can still do a smaller jump if you press B quickly.