Swarm of Obsidian Spiders

Medium swarm of Tiny Monstrosities, neutral

Armor Class 14
Hit Points 97 (15d8 + 30)
Speed 20 ft., climb 20 ft.

18 (+4) 18 (+4) 14 (+2) 10 (+0) 15 (+2) 10 (+0)

Skills Perception +5
Damage Resistances bludgeoning, piercing, slashing
Condition Immunities charmed, frightened, grappled, paralyzed, petrified, prone, restrained, stunned
Senses darkvision 60 ft., passive Perception 15
Challenge 5 (1,800 XP)
Proficiency Bonus +3


  • Amorphous Demise. When the swarm is reduced to 0 hit points or has half of its hit points or fewer, there is an explosive burst of shrapnel. Each creature in a 10-foot-radius sphere centered on the swarm must succeed on a DC 13 Dexterity save or take 28 (8d6) piercing damage.
  • False Appearance. While the swarm remains motionless, it is indistinguishable from a pile of pebbles or small rocks.
  • Magic Absorption. The grounding properties of the swarm’s glassy exoskeletons give the swarm resistance to spells and magical effects.
  • Spider Climb. The swarm can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.
  • 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 obsidian spider. The swarm can’t regain hit points or gain temporary hit points.


  • Bite. Melee Weapon Attack: +7 to hit, reach 0 ft., one target in the swarm’s space. Hit: 26 (4d10 + 4) piercing damage, or 15 (2d10 + 4) piercing damage if the swarm has half of its hit points or fewer.


  • Reflective Burst. When the swarm takes damage from a spell, it can reflect some of the spell’s energy back at the spell’s caster if the swarm is within 30 feet of the caster. The caster must make a DC 13 Dexterity saving throw. On a failed save, the target takes 27 (6d8) damage of the same type as the spell that hit the swarm, or half as much damage on a successful one.


Obsidian spiders are territorial and, unusually for arachnids, highly sociable. Their nests are always underground, but often exceptionally deep below the surface. Their primary prey are the typical insects that most spiders target, though they are frequently able to take down much larger prey when working as a swarm.


Heavily camouflaged by their rocky habitats, these tiny spiders’ exoskeletons are made of a dark and glossy chitin. This substance grounds the obsidian spider against magical effects and can reflect most low-level magics, making spellcasters’ attacks backfire.

Infrequent Origins. Obsidian spiders are very rare. When magical or other energies interact with volcanic rock, it can give rise to these creatures. Stray lightning bolts, odd wild effects, and the machinations of mad wizards have all led to the birth of obsidian spiders. These accidental populations have grown over time and spread elsewhere.

Volcanic Skitterers. These small spiders live their lives among the black sands and rocks of mountainous regions and volcanic beaches. Although most common on the Material Plane, obsidian spiders are found on the Plane of Lava and volcanic areas of other planes.

Magical Grounding. Any spell that would severely harm or even kill another creature can be reduced by the innate restrictive power of the obsidian spider’s shell. The protective property of the obsidian spider’s exoskeleton can channel magical effects away from the rest of the spider. When magical power hits the spider, it may be channeled into the surrounding rock, or recirculated through its exoskeleton’s conductive surface.

Equivalent Exchange. The obsidian spider can sometimes use the magical power coursing through its exoskeleton to lash out at an unsuspecting enemy. This makes targeting obsidian spiders with spells wildly unpredictable at best, dangerous and suicidal at worst.

Resilient Delvers. Obsidian spiders make their nests in deep caverns and forgotten places. Unlike other species of delving creatures who might stumble upon forgotten ruins or lost arcane powers, the obsidian spider is able to withstand most magical traps or long-lasting effects. Because of this, intrepid subterranean adventurers often find obsidian spiders in long-forgotten places where many other creatures would fear to tread.

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Affinity Torus Copyright 2022 Dias Ex Machina Author Chris Dias

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