Details: Immediately after Lillie's departure and the ending of the main game, your player will return home. Not one day after her leaving for Kanto, Hau stops by to inform you that he will now be heading to train at The Battle Tree. Instinctively, you go to follow your friend.
But as you exit your home, a strange and mysterious man stops you. He confirms your identity as the first Alola Region Champion, and then he gives you the enigmatic card. He tells you that you should read it, before running off at a heightened pace. If you go into your bag and take a look at the card, it asks you to join someone at the hotel on Route 8.
This officially begins the Ultra Beasts Storyline, but only if you decide to go to said hotel first.