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). Modifying the game speed via Special Smash will also affect the Stage Morph timer, despite the Match timer being unaffected.
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. While a morph is occurring, both stages will be frozen until the morph is complete. If a fighter is not within the new stage's blast lines when the morph concludes, they will be KOed immediately.
Custom stages can't be selected if Stage Morph is on. Additionally, camera controls are disabled from just before the morph occurs to just after it finishes, with camera controls being exited automatically if the frame-by-frame option is used.
In competitive play
Stage morph is universally banned in tournaments, due to its disruptive effect on gameplay, the unpredictability of occurrences, and the rising and falling of the stages being potentially exploited for gimps.