Out of the many iconic monsters from TTRPGs, there is one that stands out the least, especially in a dark dungeon room. Do you see which one I'm talking about ? Of course not, because it's transparent.
Gelatinous cubes are a staple of dungeon building, the epitome of the trap monster. In a metaphorical way, they're the equivalent to our adventurers, who delve through ancient ruins to find power to gather -- the cube just consumes what it finds, before finding a nice cubic hole to sleep in, rather than a tavern.
The problem with trap monsters is that most if not all of their value comes in the first second of encountering them. When the cube falls on you, or the cloaker reveals itself, or the roper snatches you up from above. Those are fun moments, don't get me wrong, but there is something unsatisfying to me about making a whole creature for that end. Once it's been revealed, the monster is slowly whittled down, doing little other than trading blows. At the end of the encounter, what do the players remember ? Mostly that initial surprise, while the entire rest of the encounter is simply going through the motions.
I want to change that. To that end, meet the Rolling Gelatinous Cube, aka Rollatinous cube, aka Rollie for friends and family : a more menacing version of its mundane kin, who keeps the feeling of a trap monster, but becomes a real encounter once sprung.
Arena and tactics
The arena for this battle can be relatively open, although more enclosed spaces are still much better to make use of the cube's abilities. You don't have to run this one in a tight corridor though, even a larger room works ! I do recommend not having too much open space, and especially not having too many high points like trees in the arena, since the cube still, by nature, has very little reach and ability to hit ranged characters.
The cube has its new Absorbent Mass ability to reduce damage from range and threaten ranged characters. As its weakness is usually its extremely slow speed, I've added the Rolling Cube ability, which gives it more mobility ; it's telegraphed, so your players can plan around it and try to help each other move out of the way if necessary. This should provide the cube with enough mobility as the fight goes on to not have the battle become static.
Finally, the cube gets a little wild as it gets to half hit points, granting it a new damaging aura and more mobility. At this point, it can catch up to almost anyone if they don't take care of slowing it down.
The cube deals most of its damage just by existing, so we don't need legendary actions for this one. That makes running the cube pretty simple, too, so your turns should be quick. Time to rack up the tension and get the pace rolling !