Large ooze, unaligned

Armor Class 15 (natural armor)
Hit Points 115 (11d10 + 55)
Speed 20 ft., swim 20 ft.

16 (+3) 5 (–3) 20 (+5) 1 (–5) 6 (–2) 1 (–5)

Damage Vulnerabilities cold
Damage Resistances bludgeoning, piercing, and slashing from nonmagical attacks
Damage Immunities acid, fire Condition Immunities blinded, charmed, deafened, exhaustion, frightened, prone
Senses blindsight 60 ft. (blind beyond this radius), passive Perception 8
Challenge 5 (1,800 XP)
Proficiency Bonus +3


  • Amorphous. The gearmass can move through a space as narrow as 1 foot wide without squeezing.
  • Clanging Mass. The gearmass is filled with metal objects that clang, scrape, and clink together as it moves. When it moves, it has disadvantage on Dexterity (Stealth) checks to stay silent until the start of its next turn.
  • Ooze Nature. The gearmass doesn’t require sleep.
  • Rust Metal. Any nonmagical weapon made of metal that hits the gearmass corrodes. After dealing damage, the weapon takes a permanent and cumulative –1 penalty to damage rolls. If its penalty drops to –5, the weapon is destroyed. Nonmagical ammunition made of metal that hits the gearmass is destroyed after dealing damage.


  • Multiattack. The gearmass makes two Gear Slam or Cog Toss attacks. If it hits one creature with both Gear Slam attacks, the target is grappled (escape DC 14), and the gearmass uses Engulf on the target.
  • Gear Slam. Melee Weapon Attack: +6 to hit, reach 10 ft., one creature. Hit: 12 (2d8 + 3) bludgeoning damage.
  • Cog Toss. Ranged Weapon Attack: +6 to hit, reach 20/60 ft., one creature. Hit: 12 (2d8 + 3) bludgeoning damage, and the target must succeed on a DC 14 Strength saving throw or be knocked prone.
  • Engulf. The gearmass engulfs a Medium or smaller creature grappled by it. The engulfed target is blinded, restrained, and unable to breathe, and it must succeed on a DC 14 Constitution saving throw at the start of each of the gearmass’s turns or take 14 (4d6) acid damage. Any nonmagical ferrous metal armor, weapons, or other items the target is wearing corrode at the start of each of the gearmass’s turns. If an object is either metal armor or a metal shield being worn or carried, it takes a permanent and cumulative –1 penalty to the AC it offers. Armor reduced to an AC of 10 or a shield that drops to a +0 bonus is destroyed. If an object is a metal weapon, it rusts as described in the Rust Metal trait. If the gearmass moves, the engulfed target moves with it. The gearmass can have only one creature engulfed at a time.


A cacophony of banging metal and burbling pops echoes down the corridor before a pile of metal junk encased in a greasy bubble turns the corner and seeps down the hall.

A gearmass appears as a shambling ooze full of metal junk. It collects and carries discarded bits left by tinkers, machinists, and dead constructs, slowly feeding off the rust its acid promotes in the metal. Gearmasses are not stealthy, nor quiet in the least, a saving grace for dungeon delvers encountering them. However, all too often, the metal clanging racket is so confusing that by the time curious adventurers recognize their quarry, it is too late.

Lazy Mages are the Worst. The first gearmass was crafted by a wizard who wanted the cleaning power of a gelatinous cube combined with the scouring potential of a rust monster.

Unfortunately, the oddball creation escaped during an impromptu encounter with some zealous heroes. Unchecked and uncaged, the first gearmass quickly proliferated. A few daring wizards have since captured and trained the offspring, but the risks of keeping such creatures in laboratories and workshops quickly become apparent as the creatures devour all nearby metal.

Section 15: Copyright Notice

Tome of Beasts 3 © 2022 Open Design LLC; Authors: Eytan Bernstein, Celeste Conowitch, Benjamin L. Eastman, Robert Fairbanks, Scott Gable, Basheer Ghouse, Richard Green, Jeremy Hochhalter, Jeff Lee, Christopher Lockey, Sarah Madsen, Ben Mcfarland, Jonathan Miley, Kelly Pawlik, Sebastian Rombach, Chelsea Steverson, Brian Suskind, Mike Welham

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