Your investigation keeps looping back to a certain street deep within the city slums. So many people have gone missing on this little patch of cobblestone. Yet, no evidence of foul play has ever been left behind, not even a spec of blood. No magic lingers in the area and it's easy to get lost just thinking on the problem there. Wait? Did that cobblestone just blink? Why are your boots sticking to the road? So many tiny, hungry, mouths.

The first cobble mimic wasn't a terrible threat. All it could really eat were mice and other small vermin. Then its spawn wasn't much trouble either. Sure there were many hungry mouths to feed, but they still couldn't do much damage. That and a bitten toe could be written off as a stubbed one. However, once they reached colony size, then the blood started to really fly. The swarm slows people down as they move, can choose if they stick them in place or not, and quickly reduces a creature to slurry with dozens of slapping tendrils. For foes standing outside the swarm, it can launch a volley of surrounding debris to bring it down. When a communal meal is done, dozens of small acid glands fire at once to wash the area and make it a pristine road once more.

Swarm of Cobble Mimics

Large Swarm of Tiny Monstrosities (shapechanger), neutral

Armor Class 13 (natural armor)

Hit Points 102 (12d10 + 36)

Speed 15 ft

Str 18 Dex 9 Con 16 Int 7 Wis 14 Cha 8

Skills Stealth +5

Damage Resistances bludgeoning

Damage Immunities acid

Condition Immunities prone

Sensesdarkvision 60 ft, passive Perception 12

Languages -

Challenge 5 (1,800 XP)

Shapechanger.

The mimic can use its action to polymorph into a collection of cobblestones or back into its true, amorphous form. Its statistics are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.

Adhesive (Object Form Only).

The mimic adheres to anything that touches it. A Huge or smaller creature adhered to the mimic is also grappled by it (escape DC 15). Ability checks made to escape this grapple have disadvantage. The mimic can activate and deactivate this ability as a bonus action.

False Appearance (Object Form Only).

While the mimic remains motionless, it is indistinguishable from an ordinary collection of cobblestones.

Grappler.

The mimic has advantage on attack rolls against any creature grappled by it.

Road Less Taken (Object Form Only).

Creatures moving over the swarm in object form consider it difficult terrain. However, a creature must succeed on a DC 13 Wisdom save to notice this.

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 cobblestones. The swarm can't regain hit points or gain temporary hit points.

Actions

Multiattack.

The swarm makes three Pop Rocks attacks.

Pseudopods.

Melee Weapon Attack: +7 to hit, reach 0 ft., all targets in the swarm's square. Hit: 17 (3d8 + 4) bludgeoning damage. If the mimic is in object form, the target is subjected to its Adhesive trait.

Bites.

Melee Weapon Attack: +7 to hit, reach 0 ft., all targets in the swarm's square. Hit: 6 (1d4 + 4) piercing damage plus 3 (1d6) acid damage.

Pop Rocks.

Ranged Weapon Attack: +7 to hit, range 15/30 ft., one target. Hit: 9 (2d4+4) bludgeoning damage.

Acid Wash (Recharge 6).

All creatures in the swarm's squares and within 5ft of the swarm must succeed on a DC 14 Dexterity save or take 17 (5d6) fire damage.