Can a Gelatinous Cube's Engulf be stopped by the Sentinel feat?

Question

The first part of the Gelatinous Cube’s Engulf reads:

Engulf. The cube moves up to its speed. 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 Dexterity 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.

The first benefit of sentinel reads:

Whenever you hit a creature with an opportunity attack, its speed drops to 0 for the rest of the turn. This stops any movement they may have been taking.

Now, normally, a player character cannot make an opportunity attack against an engulfing cube: The normal 5 feet attack reach means they must be in the engulf line; if they fail the saving throw, they’re engulfed; otherwise they are pushed out of the attack reach.

However, using a long weapon (10 feet reach) means that they can potentially get an attack of opportunity without ever being in the line, or after the character has been pushed.

Making a successful opportunity attack will definitely drop the cube’s speed to zero. I am also pretty sure that engulfing counts as willingly moving out of reach (correct me if this assumption is wrong), however, will sentinel stop the engulfing? The wording of sentinel seems to imply there be a connection between the speed reduction and stopping movement being taken and I am unsure if engulf counts for this.

Clarification:I am asking about the general possibility of stopping the cube, not specifically to avoid being engulfed. It seems however, that if stopping the cube like this works, that polearm master would make it possible to avoid being engulfed.

Answer 1

Yes, Sentinel can stop Engulf

Sentinel stops a creature from moving when you hit it with an opportunity attack. Specifically, "its speed drops to 0" and "this stops any movement they may have been taking."

Engulf uses movement and speed. It says "the cube moves up to its speed." This is quite clearly movement. This provokes an opportunity attack. To quote PHB p.195,

You can make an opportunity attack when a hostile creature that you can see moves out of your reach. ... You can avoid provoking an opportunity attack by taking the Disengage action. You also don't provoke an opportunity attack when you teleport or when someone or something moves you without using your movement, action, or reaction.

Engulf is movement which uses the cube's action, so provokes an opportunity attack. This occurs "right before the creature leaves your reach."

By Sentinel, if you hit with the opportunity attack, the cube stops moving. The gelatinous cube has nothing which overrules this. With Engulf, it only does its stuff if the cube enters another creature's space. Because the cube cannot move, it cannot enter another creature's space. Therefore, Engulf is interrupted.

Obviously, Sentinel does not undo any Engulfing the cube had already done before you made your opportunity attack. But the specific bit of movement (and thus Engulf) which provokes the opportunity attack (typically, when the cube tries to leave your reach) and any movement thereafter will be prevented.

Answer 2

Sentinel can stop Engulf, but not if the PC with Sentinel is the one being Engulfed (usually)

BBeast's answer is correct, but leaves out that a Cube using Engulf doesn't leave the target's reach, and therefore doesn't typically provoke an opportunity attack. However, the third benefit of Sentinel could let you stop it after it Engulfed someone else while in your reach - or combining Sentinel with Polearm Master could let you stop it cold from 10 ft away. There are probably other methods of getting OAs against targets that don't leave your reach, which will work too.

So while Sentinel will let you stop a Cube rushing past you to Engulf your teammate, it doesn't protect you - at least in normal situations or without certain other effects.