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Marrow-Sucking Swarm - AI-generated fantasy Monster

Marrow-Sucking Swarm

Necromancers often tolerate these swarms in their ossuaries because the insects keep the bones clean of rot while acting as a natural deterrent against clumsy intruders.

Monsters
Medium (Swarm of Tiny Beasts) swarm · Medium · Uncommon

Marrow-Sucking Swarm

A writhing carpet of translucent, necrophagous centipedes that nest within the porous cavities of skeletal remains in mass graves.
AC12
HP45/45
CRMedium
Init
STR
DEX
CON
INT
WIS
CHA
ConditionsNone

Combat

AC 12 · HP 45 · Medium

Marrow-Sucking Swarm

Medium (Swarm of Tiny Beasts) swarm (tiny beasts), unaligned
Armor Class12 (natural armor)
Hit Points45 (7d8 + 14)
Speed
30 ft.,
Climb 30 ft.
STR
DEX
CON
INT
WIS
CHA
Saving ThrowsDexterity
SkillsStealthPerception
Damage Resistances
Bludgeoning, Piercing, and Slashing from nonmagical attacks
Immunities
Charmed,
Frightened,
Grappled,
Paralyzed,
Petrified,
Prone,
Restrained,
Stunned
Weaknesses
Fire,
Area of Effect Spells
Senses
Blindsight 30 ft.,
Passive Perception 12
Languages
None
ChallengeMedium
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 insect. The swarm can't regain hit points or gain temporary hit points.
Batten Down. While the swarm is within the space of a pile of bones or a mass grave, it has resistance to all damage except psychic, force, and radiant damage. Additionally, its AC increases to 15.
Bone Camouflage. While the swarm remains motionless in a bone-filled area, it is indistinguishable from normal decay and moss.

Actions

Marrow Draining Bites. Melee Weapon Attack: to hit, reach 0 ft., one creature in the swarm's space. Hit: 10 () piercing damage, or 5 () piercing damage if the swarm has half of its hit points or fewer. The target must succeed on a Constitution saving throw or take 7 () necrotic damage as the insects begin to liquefy their marrow.
Find the Gap. The swarm attempts to crawl into the armor or clothing of a creature in its space. The target must make a Dexterity saving throw. If the target is wearing Heavy Armor or has a Dexterity score of 11 or lower, it has disadvantage on this saving throw. On a failure, the swarm deals 14 () piercing damage and the target is considered Poisoned by the intense pain until the end of their next turn.

Reactions

Scurry Home. When the swarm takes damage from a source it can sense, it can move up to half its speed toward the nearest pile of bones without provoking opportunity attacks.
Role
Environmental Hazard/Ambush Predator
Built for
3

Notes for the game master

Remember that the swarm can occupy the same space as the PC
The extra damage from 'Find the Gap' represents the centipedes reaching the vulnerable marrow through armor joints

Tactics

  1. Opening

    The swarm starts hidden in bone piles, waiting for a PC with heavy armor to walk over them.

  2. Mid-fight

    It uses 'Find the Gap' on the least agile party member while remaining in the bone pile to benefit from 'Batten Down'.

  3. End of the fight

    If reduced to low HP, it tries to spread out across multiple bone piles to confuse the party.

  4. Retreat

    The swarm will not leave the area of the mass grave. If forced out, it will spend its turns attempting to move back to the bones.

How it fights

Prioritize targets with the lowest Dexterity modifiers
Stay stationary in bones as much as possible for the resistance boost
Ignore agile targets like Rogues if a Paladin or Fighter is nearby

Who it attacks first

Creatures in Plate or Chain Mail
Creatures with negative Dexterity modifiers
Downed or prone creatures

Works well with

Skeletons (who provide mobile 'Batten Down' sites for the swarm)
Moss Monsters (who slow down PC movement)

How to beat it

Fireball or Burning Hands
Characters with high AC but also high Dexterity
Magical area effects that bypass the Batten Down resistance

Ways to avoid a fight

The swarm can be distracted by throwing fresh meat or high-calcium bone meal away from the party's path
Casting Speak with Animals might reveal they are just hungry and territorial

Ecology

A mass of thousands of pale, nearly translucent centipedes. Each insect has a needle-like proboscis and visible, pulsing red digestive tracts filled with stolen marrow. They move with a sickening, liquid sound over and through piles of damp, moss-covered bones.

Behavior

They remain dormant within bone piles until they sense the vibrations of heavy footsteps. They prioritize targets that move slowly or wear clunky gear, seeking the gaps in armor to reach the soft skin beneath.

How it lives

These insects are cleaners of the dead, but they have evolved to accelerate the process by liquefying the marrow of the recently deceased or the living. They are often found in the company of mosses that thrive on the slurry they leave behind.

Lair

Mass graves where bone density is high, providing them with both food and armor.

Habitat
Grimy dungeons, mass graves, mossy crypts, and ancient battlefields.

Signs that it is near

Hollowed-out bones that feel strangely light
A faint, rhythmic clicking sound echoing from underfoot
Trails of viscous, yellowish fluid on damp moss
Skeletons that appear to 'shiver' when approached

Tags

swarm,
dungeon,
hazard,
undead-adjacent

Lore

Necromancers often tolerate these swarms in their ossuaries because the insects keep the bones clean of rot while acting as a natural deterrent against clumsy intruders.

What drives it

Survival and the consumption of mineral-rich bone marrow.

After the fight

The party finds their bones feel strangely cold and heavy if they were bitten, requiring a long rest or Lesser Restoration to feel normal again.

Variants

Ancient Marrow-Suckers: Larger swarm that deals more necrotic damage
Moss-Crested Swarm: Can also inflict the Restrained condition via sticky moss

Hooks

Plot hooks

The local gravedigger refuses to enter the lower pits because the skeletons have started to 'hum' and 'bite'
A scholar needs a sample of the 'translucent ichor' produced by these insects to cure a bone-wasting disease

How to use it

As a trap in a room filled with difficult terrain (bone piles)
As a secondary threat during a fight with a larger undead creature like a Wight

Treasure

Occasionally, jewelry remains in the marrow-scoured bones, as the insects don't eat metal
Centipede Ichor (can be used as a component for necrotic potions)

Media

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