The corridor ahead looks empty. Your torchlight shows stone floor, stone walls, stone ceiling. Then you notice the torch does not reflect off the floor quite right — not stone, something else — and the not-stone is already around your boots.
What a Gelatinous Cube Is #
A gelatinous cube is a 10×10×10 ft block of transparent jelly that moves through dungeon corridors eating everything biological and collecting everything it cannot dissolve. It is not intelligent, not aggressive in any behavioral sense — it simply moves through the dungeon on its established path, consumes what it encounters, and continues.
The ecology role: “A gelatinous cube moves through the rooms and corridors of a dungeon, sweeping the halls clean of all living and dead material.” The gelatinous cube is the dungeon’s janitor — it removes corpses, organic waste, and dead creatures from corridors, leaving them clean. A dungeon with gelatinous cubes has notably cleaner corridors than one without. Experienced dungeon explorers know this: an unusually clean corridor may indicate a cube’s patrol route.
The carried treasure: The cube collects items it cannot dissolve — coins, gems, weapons, armor — inside its body. These items are visible through the transparent jelly but cannot be retrieved without entering the cube or killing it. This is why the cube has Treasure Type V: accumulated from everything it has absorbed.
Core Statistics #
| Stat | Value |
|---|---|
| Hit Dice | 4d8* (avg 18 HP) |
| AC | 12 |
| AV | 0 |
| HR | +4 |
| FR | +4 |
| FD | 14 |
| Move | 60 ft (20 ft encounter) |
| Attacks | 1 touch: 2d4 + paralysis |
| No. Appearing | 1 (0 standard lair; up to 1d4 in cube lair) |
| Save As | Fighter 2 |
| Morale | 12 |
| Treasure | Type (V) — carried inside body |
| Alignment | Neutral |
| Intelligence | None (INT 0) |
| XP | 125 |
AC/AV Reasoning #
RC original is AC 8 (descending) = Ascending AC 12. The gelatinous cube is transparent jelly — it has no structural defense.
- AC 12 — The cube is hard to strike effectively not because it is armored but because striking jelly is genuinely difficult. A blade that hits the cube may pass through the outer layer without producing the impact needed for meaningful damage. The AC reflects the bizarre physical properties of semi-liquid material under attack
- AV 0 — No protective structure whatsoever. When a weapon strike does connect effectively, it deals full damage. The cube’s defense is entirely AC-based (the difficulty of hitting jelly meaningfully)
- The fire/weapon exception: Fire and weapons deal full damage. Cold and lightning do not harm it (see Immunities below)
Skill Slots (4 total — 4* HD = 4 base + 1 asterisk; INT 0) #
| Slot | Skill / Ability | Notes |
|---|---|---|
| 1 | HR Investment (Basic) | HR +4; the touch attack is simply contact — the cube engulfs anything it moves into |
| 2 | Paralytic Touch (innate, special) | 2d4 damage + Save vs. Paralysis on contact. The asterisk |
| 3 | Near-Invisibility (innate) | Surprise on 1–4 (d6). Hard to detect in torchlight. Full mechanics below |
| 4 | Elemental Resistance (innate) | Immune to cold and lightning. Fire and weapons deal normal damage |
Near-Invisibility — The Encounter’s Core Danger #
“This monster looks like a great quantity of clear jelly. It is hard to see, and it surprises often (1–4 on 1d6).”
Why It’s So Hard to See #
The gelatinous cube is transparent — torchlight passes through it rather than reflecting off it. In a stone dungeon corridor:
- The stone floor, walls, and ceiling are visible through the cube
- The cube’s own body is invisible except for a faint shimmer at its leading edge
- Items suspended inside the cube (coins, weapons, bones) may be visible but appear to be floating in air
- Any faint reflection from the cube’s surface requires catching the light at exactly the right angle
Detection Methods #
Carried items: The most reliable tell — coins or bones appearing to float in mid-air indicate a gelatinous cube. A party that notices “floating items” has time to avoid.
Surface sheen: The cube’s outer layer has a faint wet sheen. In good light (within 20 ft of a torch), a careful observer who is specifically looking for the cube may notice this. Alertness check (difficulty –4) to detect the sheen before contact.
Corner effect: Where the cube contacts a wall, there is a visible meniscus (curved junction between the jelly and stone). A character watching the walls rather than the floor ahead may notice this.
Smell: The cube produces a faintly chemical, sweet smell — organic dissolution in progress. Nature Lore or Healing check (difficulty –3) to recognize the smell as biological dissolution before contact.
No advance warning: A party not actively looking for the cube will simply walk into it. The surprise roll of 1–4 on d6 (approximately 67% surprise rate) means most encounters begin with at least some party members already inside or against the cube’s surface.
Paralytic Touch — Complete Mechanics #
The Attack #
Any successful hit on a living creature:
- Deals 2d4 damage (avg 5)
- Requires a Save vs. Paralysis
Failed save: Target is paralyzed for 2d4 turns (20–80 minutes) unless magically cured.
Paralyzed condition:
- Cannot move or take any actions
- Is inside or in contact with the cube (the cube has not moved away — it continues to absorb the paralyzed character)
- Takes automatic hits each round (see below)
Automatic Hits Against Paralyzed Targets #
“An attack on a paralyzed target automatically hits (only a damage roll is needed).”
Mechanics: Each round the cube deals 2d4 damage to a paralyzed character in contact with it — no HR roll, no AC defense. The character simply takes the damage. The paralysis save is not repeated each round — the character is paralyzed for the full 2d4 turns regardless of subsequent damage.
The math: A character paralyzed inside the cube for 2d4 turns takes 2d4 damage per round (avg 5) for avg 12–30 rounds. At avg 5 HP per round over 20 rounds = 100 HP of automatic damage. No character survives 2d4 turns inside a gelatinous cube without magical intervention.
Rescue: A paralyzed character being absorbed by the cube must be rescued before the paralysis ends — because by then they are dead. Options:
- Pull them out of the cube (FR vs. FD 14 by a companion, then carry the paralyzed body away)
- Cast Remove Paralysis or equivalent cure on the character while they are still being absorbed (the character can then move themselves out)
- Kill the cube (the jelly stops absorbing when the cube dies; the character’s body can be recovered)
Multiple Targets #
The cube can engulf an entire corridor width (10 ft). Any character in a 10 ft wide corridor when the cube makes contact may be simultaneously touched. Each contact requires a separate HR roll and separate paralysis save.
In a 10-ft-wide corridor, the cube fills it completely — there is no room to step aside unless the party can move backward faster than the cube advances (cube speed 20 ft encounter; most characters 40 ft encounter — the party can outrun the cube at full speed, but a surprised party that has been partially engulfed may not get that option).
Immunities #
“A gelatinous cube may be harmed by fire and weapons, but not by cold or lightning.”
Cold Immunity #
Cold damage has no effect on the gelatinous cube. The cube’s chemistry is apparently cold-stable — freezing or chilling the jelly does not damage it.
Practical note: A party whose primary magical offensive resource is cold-based (Cone of Cold, Ice Storm) finds their spells completely wasted against a gelatinous cube. This makes the cube a specific counter to cold-specialist parties.
Lightning Immunity #
Lightning has no effect on the gelatinous cube. The jelly apparently conducts electricity harmlessly through its mass and into the floor.
Practical note: A Lightning Bolt passing through a gelatinous cube would deal full damage to anything inside the cube (a paralyzed character, for example) while doing nothing to the cube itself. Potentially very dangerous to use in the presence of a cube that has absorbed a party member.
Fire Vulnerability #
Fire deals full normal damage. The cube’s organic chemistry burns effectively. Flaming oil, Fireball, Burning Hands, and fire-based attacks all deal normal damage.
Fire rescue caveat: If a paralyzed party member is inside the cube when the party uses fire on it, the character inside also takes the fire damage (they are in the cube, not protected by it from external fire). Rescuing a character inside a cube with fire requires accepting that the rescue may kill the person being rescued if the cube has insufficient HP remaining.
The Dungeon-Cleaning Ecology #
Patrol Routes #
Gelatinous cubes follow established patrol routes through their dungeon — moving through corridors in a consistent pattern, cleaning as they go. “It moves through rooms and corridors of a dungeon, sweeping halls clean.”
Route indicators:
- Unusually clean corridors (no dust, no bones, no organic debris)
- Items that look like they are floating in air mid-corridor (suspended in transparent jelly)
- No small creatures (rats, insects) in areas the cube patrols — they are absorbed
- A sweet/chemical smell in otherwise clean corridors
Patrol frequency: DM determination — a cube might complete its circuit in 1d4 days, leaving behind clean corridors that accumulate fresh debris until the next pass.
What It Collects #
Absorbed (destroyed):
- Biological tissue — flesh, bone (eventually), organic material
- Dead creatures (corpses are consumed on its path)
- Living creatures (absorbed and dissolved)
- Leather, cloth, wood — eventually dissolved
Carried (cannot dissolve):
- Metal (coins, weapons, armor, tools)
- Gems and minerals
- Magical items — may or may not be dissolved (DM determination; magical items often resist the dissolving)
- Glass and ceramic
The treasure implication: The cube’s body contains whatever metallic and mineral items it has collected on its patrol. Looking through the transparent jelly reveals the contents. A cube that has been patrolling a dungeon for years may contain significant accumulated treasure from dozens of absorbed creatures and cleared rooms.
No Lair in the Standard Sense #
“No. Appearing: 1 (0)” — the zero in parentheses means no standard lair encounter. The cube is encountered in corridors, not in a room where it “lives.” However:
“The lair of these strange monsters may contain 1d4 cubes (each with treasure type V).” — A specific cube lair (a room the cubes have claimed or returned to) may hold multiple cubes. This is not a standard lair entry but a specific dungeon feature the DM places deliberately.
Adults split: “Adults split into two fully grown cubes” — there are no young gelatinous cubes. When a cube becomes large enough, it splits into two full-sized cubes. This is why a cube lair has multiple cubes — they have accumulated and reproduced.
Encounter Notes #
The corridor approach: The party is moving through a dungeon. The corridor ahead looks clean — notably clean, actually. Unusually clean for a dungeon this old. A character who has encountered gelatinous cubes before or who succeeds on a Nature Lore check knows that clean corridors may indicate a cube patrol. The party debates whether to proceed or find another route.
The floating coin: Twenty feet ahead, a gold coin appears to be floating three feet off the ground. It is not floating — it is inside a gelatinous cube. A party that investigates the floating coin without suspecting what it means walks into the cube’s surface.
The partial engulfment: The party rounds a corner. The two characters in front are immediately against the cube’s surface (surprise 1–4 on d6). They take 2d4 damage and must save. One succeeds, one fails. The failed character is now paralyzed and will begin taking automatic 2d4/round next round. The party has limited rounds to pull the paralyzed character out or kill the cube (18 avg HP — not difficult to kill) before the paralysis kills their companion. Standard lighting tools (fire) may be available but will harm the absorbed companion.
The lair of cubes: A specific dungeon chamber has 1d4 gelatinous cubes. The party entered from one side. The cubes fill the corridor in multiple places. Significant collected treasure is visible inside each cube’s body — but reaching it requires dealing with the cubes while they paralyze party members one by one.
CR 4 justification:
- 4* HD = CR 5 theoretical
- Reduced to CR 4: Slow movement (60 ft — party can outrun it), INT 0 (no tactics), fire weakness (very common party resource), Morale 12 but no ability to retreat (it will just get killed by fire eventually)
- Against a party without knowledge of the cold/lightning immunity: treat as CR 3 (cold/lightning specialists find the cube trivially easy if they’re using cold magic and then discover it doesn’t work)
- Against a party using Lightning Bolt on a cube that has absorbed a companion: treat as CR 5 (the rescue dilemma elevates the danger)
