After defeating all five Titans you'll have to fight against your friend Arven.
Arven's research into healthy recipes for Pokemon has led him to seek out rare ingredients called Herba Mystica, which are said to immediately restore health once eaten. Herba Mystica are extremely rare herbs that can only be found in Paldea. They are guarded by Titan Pokemon, which are bigger and stronger than ordinary Pokemon- making these rare herbs even more difficult to obtain. What's more, a number of these Titan Pokemon have already been sighted in the region. Since Arven isn't good at Pokemon battles, he'll be asking you to help him out - and will even offer to treat you to some of his hand-made dishes if the two of you succeed in obtaining these rare herbs. Drive back tough opponents and gather the Herba Mystica together with Arven!