Sea of Clouds
From the Super Mario Wiki
Kirby and the rescued princess, still riding Kirby's Warp Star, are forced to land on the Halberd after it crashes into them. They explore the ship until they see an Arwing get shot by the Halberd. Kirby and the princess are shot, also. They fall off the ship and explore the Sea of Clouds until they find a yellow door.
Stage Clear Reward
Songs played in Subspace Emissary stage (in order of appearance)