Arven asks you to investigate the Open Sky Titan in West Province (Area One).
How to Find the Open Sky Titan
Go northwest from Cortondo and you eventually reach the "West Province (Area One) - Central" Pokémon Center. Go northwest from there to the Open Sky Titan destination on the map.
However, you will find that the Open Sky Titan is dropping boulders on the path, so you will need to carefully ride Koraidon/Miraidon up the slope and avoid letting the boulders hit you.
Battle the Open Sky Titan
Once you reach the top of the area, you will automatically begin the battle against Bombirdier, the Open Sky Titan. After you lower its HP, there will be a cutscene, and you and Arven will have a double battle against it with his level 19 Nacli.
After you defeat it, there will be a cutscene, and you will get the Bitter 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 move across water.