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After obtaining the National Dex, another Pokemon is available to unlock. This Pokemon is the Lunar Pokemon, Cresselia. Like Mepsrit, you cannot find Cresselia through the Pokedex. You must obtain the mapping app from the Poketch President in Jubilife City.

In order to unlock her, you must go to Canalcave City. By the dock is a house with a sign in front of it. Go inside and speak to the woman. Return outside and speaking with the sailor. He will ask you to find an item on Fullmoon Island. When you go there, it is simple to get through and you will enter a small clearing. Speak with Cresselia and she'll fly off and leave behind an keyitem called "Lunar Wing." Give it to the sailor and return to Canacave.

Now, Cresselia will appear around Sinnoh much like the Beasts and the Latis did in GSC/RSE. With the Map App you got in Jubilife, put a high level and fast Pokemon with the move "Mean look" first. Walk around some routes until Cresselia appears on the map. Head to that route and walk until you find it. Use Mean Look off the bat, and then switch to your attacking Pokemon. Weaken it to RED HP, and put it to sleep if you can. If Cresselia runs before the Mean look, repeat the process and hope Mean Look hits first.