Arven asks you to investigate the False Dragon Titan in Casseroya Lake.
How to Find the False Dragon Titan
The False Dragon Titan can be found on the eastern island in Casseroya Lake, which is northwest of Medali.
Battle the False Dragon Titan
On the eastern island in Casseroya Lake, look for a large Tatsugiri (sushi fish) that is saying "Taitan". Talk to it, and the False Dragon Titan will attack. Its moves are Body Slam, Aqua Tail, and Water Pulse.
After you lower its HP, it will swim away. Go to the north side of the island in the center of the lake. You can see the location on your map. When you go there, you will see a Tatsugiri on a ledge in shallow water. Approach that Tatsugiri and there will be a cutscene. Afterward, you and Arven will have a double battle against the Pokémon. Its moves are Body Slam, Aqua Tail, Water Pulse, and Order Up. Arven has a level 55 Greedent.
After that, you and Arven will have a double battle against Tatsugiri. Its moves are Dragon Pulse, Icy Wind, Muddy Water, and Taunt.
After you defeat it, there will be a cutscene, and you will get the Spicy 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 climb up vertical surfaces. While riding Koraidon/Miraidon, press B to grab onto a wall that you are facing, then move the left stick to move in any direction on the wall. Press B again to let go of the wall.