Walkthrough: After defeating the final Trial and having now reached the Alter of the Moone, it is up to you to summon Lunala in hopes of being able to access Ultra Space to find Guzma and Lusamine. As you walk into the Altar, Lillie will suggest the two of you play one flute each, on opposite sides of the Altar.
As both of you begin to play in your given spots, the Altar will begin to light up. However, as this is happening, Lillie's bag will begin to move and Cosmoem (Nebby) will break free from her bag. It will float to the center of the Altar, and it will soon glow and actually evolve into none other than the Cover Legend, Lunala. After evolving, Lunala will beam open a portal for both you and Lille to travel to Ultra Space with it.