With a wall in the Active spot, the Legendary/Mythical duo sit back, attaching energy to themselves, with Darkrai's Nightmare Aura ability chipping away at the opponent's Active Pokemon's HP.

This is the opposite of its counterpart, the literal nightmare fuel that is Darkrai ... pokemon.com) When you reach the island, you’ll find Cresselia awaiting you in a glade. The Legendary ...

Although you don't need to complete every single task in the game, you still need to reach the end of the main plot to unlock the pair of Pokemon that are classified as "legendary." For Darkrai ...