This extraplanar swarm is composed of countless 6 inch diameter spiders comprised of Astral goo, called ectoplasm, hardened into chitin.
Astral Pests. These spiders come from the depths of the Astral Plane where vast ectoplasmic webs cover the area, unanchored but given form through continual motion. These webs ensnare unwary travelers, signaling an endless stream of spiders to devour the unfortunate creature.
All Consuming. The swarm will quickly overcome and devour living creatures, transforming their remains into ectoplasm, from which more of the spiders are created, adding to the swarm. When a swarm grows large enough, it splits in two, with each swarm voraciously seeking new prey to consume so it can further reproduce.
Medium swarm of Tiny monstrosities, unaligned
Armor Class 16 (natural armor)
Hit Points 40 (9d8)
Speed 20 ft., climb 20 ft.
Damage Resistances bludgeoning, piercing, slashing
Condition Immunities charmed, frightened, grappled, paralyzed, petrified, prone, restrained, stunned, unsettled
Senses darkvision 60 ft., passive Perception 10
Challenge 2 (450 XP) Proficiency Bonus +2
Ectoplasmic Cocoon. A creature damaged by the swarm is restrained by ectoplasmic webbing. A creature can use its action to make a DC 14 Strength saving throw, freeing the restrained creature on a success. The webbing can also be attacked: it has AC 10 and 20 hit points. The swarm can only restrain one creature at a time.
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 spider. The swarm can’t regain hit points or gain temporary hit points.
Bites. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 14 (4d6) force damage, or 7 (2d6) force damage if the swarm has half of its hit points or fewer.