After you caught Dialga and Palkia, which closed the Space-Time Rift that had opened in Hisui's sky, Volo, the merchant of the Ginkgo Guild, asked you to find the 18 Arceus Plates. That's when Adaman, the leader of the Diamond Clan, came to find you with news that don't sound good: Melli, one of his Wardens, needs your help at Moonview Arena in Coronet Highlands because Lord Electrode is acting strangely, like scared of something. So you decide to go see what's happening.

Once at Moonview Arena, you find Melli comforting Lord Electrode, the usually arrogant and loud Warden acting surprisingly soft with his Lord Electrode. Once he notices you, Melli turns to you.

Melli: Ah, you're here! I don't know what's going on, but my dear Lord Electrode seems scared of something that's in his arena! Could you go check what's going on?

He says, pointing towards the arena in which, from where you stand, you can already see the legendary Pokémon Cresselia, who's definitely not supposed to be here.