Gray Goo

Large swarm of Tiny constructs, unaligned

Armor Class 18 (natural armor)
Hit Points 130 (20d4 + 80)
Speed fly 50 ft.

23 (+5) 26 (+8) 18 (+4) 18 (+4) 10 (+0) 10 (+0)

Damage Vulnerabilities lightning
Damage Resistances bludgeoning, piercing, slashing
Damage Immunities cold, necrotic, poison, psychic; bludgeoning, piercing, and slashing from nonmagical attacks that are not adamantine
Condition Immunities charmed, frightened, grappled, paralyzed, petrified, poisoned, prone, restrained, stunned
Skills Acrobatics +13
Senses blindsight 10 ft., darkvision, low light vision, passive Perception 10
Languages understands all languages, but cannot speak
Challenge 14 (11,500 XP)


  • Disassembler Protocol. A gray goo automatically destroys it’s choice of one medium object, a 5 foot cube out of a large or larger object, or 1d6 smaller unattended objects in its space at the beginning of its turn.
  • Mechanical Nature. The robot swarm does not require air, food, drink, or sleep.
  • Swarm. The swarm can occupy another creature’s space and vice versa, and the swarm can move through any opening large enough for a Tiny creature to pass through. The swarm can’t regain hit points or gain temporary hit points.


  • Dismantle. Melee Weapon Attack: +10 to hit, reach 0 ft., one target in the swarm’s space. Hit: 71 (10d12 + 5) slashing damage. The target must succeed at a DC 18 Dexterity saving throw or a random piece of equipment that they are wearing or wielding is destroyed. If this damage reduces the target to 0 hit points, its body becomes a pile of fine gray dust.
  • Infest. Melee Weapon Attack: +10 to hit, reach 0 ft., one medium or larger target in the swarm’s space. Hit: The target must succeed at a DC 18 Constitution saving throw or the swarm infests the target. At the beginning of the infested creature’s turns it takes 100 (18d10) slashing damage. If this damage reduces the host to 0 hit points, its body becomes a pile of fine gray dust. A gray goo cannot use it’s dismantle attack or disassembler protocol abilities while infesting a host, but it can be targeted normally. A gray goo infesting a host can be expelled by any effect that cures disease.


  • Replicate. The gray goo splits into a second cloud of identical tiny robots. 1 new gray goo coalesces in an unoccupied space adjacent to the original goo. A gray goo can only use this ability after having dealt 250 points of damage with its dismantle or infest attacks. Each use of its disassembler protocol ability counts as having dealt 20 damage for this purpose.


Over millennia, explorers from across the known galaxy pass down fearful tales of a sentient, liquid-like swarm of nanites that can envelop creatures and quickly turn them to dust. Once connected to a mysterious species of abductors who used advanced fringe technology with a similar silvery sheen, these creatures gained the moniker gray goo.

Absorb and Copy. When not engaged in the consumption of material, most gray goos seem to have two driving directives: replicate and assimilate. The clouds of nanites have even hacked computer systems and technological constructs of sufficient sophistication, usually through brute force methods, to access and incorporate any useful data within. If a computer system’s countermeasures block it, the goo attempts to dismantle the console and reduce it to its essential components before stripping even those parts to a fine metallic dust.

Section 15: Copyright Notice

Mechanical Monsters (5E) © 2023, Legendary Games; Authors: Jason Nelson, Miguel Colon, Robert J. Grady, Nicholas Hite, Matt Kimmel, Michael Mifsud, James-Levi Cooke, Dan Dillon, Mike Myler, Ismael Alvarez, Jeff Lee, John Lynch.

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