The gelatinous cube possesses the extraordinary ability engulf that says
Although it moves slowly, a gelatinous cube can simply mow down Large or smaller creatures as a standard action. It cannot make a slam attack during a round in which it engulfs. The gelatinous cube merely has to move over the opponents, affecting as many as it can cover. Opponents can make opportunity attacks against the cube, but if they do so they are not entitled to a saving throw. Those who do not attempt attacks of opportunity must succeed on a DC 13 Reflex save or be engulfed; on a success, they are pushed back or aside (opponent’s choice) as the cube moves forward. Engulfed creatures are subject to the cube’s paralysis and acid, and are considered to be grappled and trapped within its body. The save DC is Strength-based and includes a +1 racial bonus. (Monster Manual 203)
Can an answer walk me through the turn of a gelatinous cube that wants to use this special ability?
I mean, so it's the cube's turn and the cube takes a move action to get nearer its foes then it takes a standard action to… do what exactly? Does it take that standard action to continue moving into the occupied squares of as many foes as its 15 ft. speed allows, potentially engulfing every foe in its path with no maximum as to the number of creatures that can be engulfed? (And hilarity ensues!) Does it take that standard action to engulf but one adjacent creature by moving into that creature's space and covering the creature, a result that seems disputed by the description of the engulf ability? Or does something else happen that I'm totally overlooking?
Note: The last cube I ran was a Gargantuan cube advanced to 24 Hit Dice partnered with a Huge advanced Monster Manual mimic (186) in a high-level adventure. However, the pair just hit and grappled foes, the cube never engulfing. While doing some reading, I was pointed at the ninth and final issue of the ex-Wizards of the Coast messageboard-sponsored Knowledge Arcana PDF e-zine that includes the prestige class adamantine chef. (For those interested, previous Knowledge Arcana issues are available by changing that issue #9 URL from 09.PDF to 08.PDF and so on, and also available is the nominal 3.5 update of the pie fiend that's issue #9's PDF Web extra.) Included in the prestige class article is a new monster that can be summoned by the adamantine chef: the death salad (I'm not even kidding)… that has the same special ability engulf as the cube. In trying to summarize the death salad's engulf ability for personal use, I realized I had no idea how the ability worked and that the cube's ability worked in an identically baffling fashion.
Also, while several 5e questions have been posed about the cube's engulf ability, only this one is really about the 3.5e cube and its engulf ability, but it's about how much damage the cube deals after it's engulfed a hapless foe rather than how the engulf ability works.
Best Answer
The move action and the Engulf's standard action are separate.
By virtue of the comment:
...it is apparently allowed to move into the spaces of opponents. If any creatures it wants to engulf is in the covered space at the end of it's move, it can then take a standard action to engulf whatever creatures are in the target area. (By changing the permeability of it's membrane, maybe?) If they succeed on the reflex save, they are instead pushed back or to the side (opponent's choice).
Creatures that fail are now "engulfed" and are thus in a legal positioning at the end of the cube's turn. Creatures that succeed on the save are "pushed back or to the side", and are also thus in a legal positioning at the end of the cube's turn.
Should it not choose to engulf, then it would be subject to the standard movement rules and have to end it's turn in a legal space.