Catch Cresselia

Catching the Psychic-type Legendary Pokemon Cresselia requires you to trigger a small side quest in Canalave City. Head there and enter Sailor Eldritch’s house (the house next to sailor who took you to Iron Island).

Talk to the little boy and his mother, then leave and talk to the sailor next to the house. He’ll take you to Fullmoon Island, where you’ll find Cresselia.

It will run, but you’ll be able to pick up the Lunar Feather it leaves behind. Take this back to the little boy to cure him, then chase after Cressselia the same way you did for Mesprit. It’s also Lv. 50 - so use the same exact method you used to catch Mesprit.