Imagine a hallway, 10 ft wide and 10 ft tall, such that the Cube completely fills it, as per its Ooze Cube trait. Immediately adjacent to the Cube are 2 Medium creatures, which span the width of the hallway by taking up 5 ft each, and immediately behind them are 2 more Medium creatures, again spanning the hallway. The hallway is clear behind this second row. On its turn, the Cube moves up to 15 ft and will attempt to Engulf all 4 Medium creatures as it travels.
How should the Engulfing be resolved?
For reference, this is the relevant text of the Cube's Engulf action (emphasis mine):
The cube moves up to its speed [15 ft]. While doing so, it can enter Large or smaller creatures' spaces. Whenever the cube enters a creature's space, the creature must make a DC 12 DEX saving throw.
On a successful save, the creature can choose to be pushed 5 feet back or to the side of the cube. A creature that chooses not to be pushed suffers the consequences of a failed saving throw.
Notably, in this situation, the Medium creatures cannot be pushed to the side (since the Cube fills the entire hallway) and it seems those in the front row cannot be pushed back (as Jeremy Crawford states "you can't willingly end your move… in another creature's space").
- The Cube moves 5 ft and attempts to Engulf the first row of creatures. They automatically fail their save, because they have nowhere to go. The Cube continues to spend movement, going another 5 ft forward and attempts to Engulf the second row, which attempt their saves as normal. Does this count as 2 uses of the Engulf action or 1 continuous use?
- The Cube moves 5 ft and attempts to Engulf the first row of creatures, which make their saves as normal. On success, the entire group is pushed back, with the first row effectively forming a moving wall between the Cube and the second row.
- As the previous possibility, but as the group retreats, the Cube continues to move the full 15 ft forward, forcing those in the front row to make up to 3 saves each as it advances. This one seems intuitively to be multiple uses of the Engulf action rather than a single continuous action.
- The Cube moves 5 ft and attempts to Engulf the first row, which make their saves as normal. On success they are pushed back through their allies, effectively swapping the rows. The new first row then makes their save against the Engulf as the Cube continues to move forward.
- The Cube moves 10 ft (through the first row) and attempts to Engulf all 4 creatures simultaneously. They all make their saves at once, and those that succeed are pushed backwards as a group, relative to where they started.