Stage Morph is a feature in Super Smash Bros. Ultimate, available as a toggle in the Rules selection, which was revealed during the Smash Bros. Direct on August 8, 2018. It allows players to choose two different stages on the stage selection screen for a match, and have these stages transform into each other mid-battle.
The frequency of the transformation can be set to Off, which disables the feature; Random, which transforms the stage at random intervals; or Every [interval], which transforms the stage at a configurable interval (with a default of 1:00).
The transition itself bears some resemblance to the transitions of Castle Siege and Reset Bomb Forest. However, instead of temporarily placing fighters on a flat walk-off surface, the terrain of the new stage rises up directly through the current stage's sinking terrain.