Gelatinous Cube
Large Ooze, unaligned
450 XP
Special Abilities
Ooze Cube
The cube takes up its entire space. Other creatures can enter the space, but a creature that does so is subjected to the cube's Engulf and has disadvantage on the saving throw.
Creatures inside the cube can be seen but have total cover.
A creature within 5 feet of the cube can take an action to pull a creature or object out of the cube. Doing so requires a successful DC 12 Strength check, and the creature making the attempt takes 10 (3d6) acid damage.
The cube can hold only one Large creature or up to four Medium or smaller creatures inside it at a time.
Transparent
Even when the cube is in plain sight, it takes a successful DC 15 Wisdom (Perception) check to spot a cube that has neither moved nor attacked. A creature that tries to enter the cube's space while unaware of the cube is surprised by the cube.
Actions
Pseudopod
Melee Weapon Attack: +4 to hit, reach 5 ft., one creature. Hit: 10 (3d6) acid damage.
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.
On a failed save, the cube enters the creature's space, and the creature takes 10 (3d6) acid damage and is engulfed. The engulfed creature can't breathe, is restrained, and takes 21 (6d6) acid damage at the start of each of the cube's turns. When the cube moves, the engulfed creature moves with it.
An engulfed creature can try to escape by taking an action to make a DC 12 Strength check. On a success, the creature escapes and enters a space of its choice within 5 feet of the cube.
Quick Reference
Running This Encounter
The Gelatinous Cube mindlessly moves through dungeon corridors in a straight line, consuming everything in its path. Use its low AC as a trap itself—it's hard to hit because of its amorphous, slippery body. Have it move 15 feet per turn and engulf creatures that fail to escape its space. Consider using it as an environmental hazard that forces the party to solve problems creatively rather than just fight.
Combat Tactics
The Cube has no tactics or intelligence—it simply moves forward and engulfs prey. Creatures inside take acid damage each turn but can attempt to cut free from within. It's best used as an obstacle or chase encounter rather than a tactical combat opponent.
Environment & Setting
Dungeon corridors exactly 10 feet wide are ideal, trapping the party between the Cube and walls. Caverns, catacombs, or prepared dungeons with straight passages maximize its threat.