Jellyfish Swarm

Large swarm of Tiny beasts, unaligned

Armor Class 15 (natural armor)
Hit Points 120 (16d8+48)
Speed swim 20 ft.

1 (-15) 13 (+1) 14 (+2) 2 (-4) 13 (+1) 1 (-5)

Saving Throws Con +5
Damage Resistances bludgeoning
Damage Immunities poison
Condition Immunities charmed, frightened, grappled, paralyzed, petrified, prone, restrained, stunned
Skills Stealth +4
Senses darkvision 60 ft., passive Perception 11
Challenge 6 (2,300 XP)

Special Traits

  • 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 jellyfish. The swarm can’t regain hit points or gain temporary hit points..
  • Water Breathing. The jellyfish swarm can only breathe underwater.


  • Mass of Tentacles. Melee Weapon Attack: +6 to hit, 0 ft. reach, two targets in the swarm’s space. Hit: 21 (6d6) poison damage. If the target was a creature it must succeed a DC 15 Constitution saving throw or be poisoned for 1 minute.


Unlike their smaller cousins, giant jellyfish are active predators that seek out prey. Capable of slithering through narrow cracks, a giant jellyfish is a horrifying beast to encounter lurking in the hold of a flooded or sunken ship. Other species of these vermin exist, as summarized on the following table- these variants often have different types of poison or other abilities like translucency or constriction.

Blood Feeders. The crimson jellyfish is a blood-drinking creature whose red color comes from the blood absorbed throughout its body after it feeds. A large fish or sea mammal encountering a pack or bloom of these creatures can be drained of blood in a matter of minutes. Fortunately, a crimson jellyfish’s bright color makes it fairly easy to avoid under well-lit conditions.

Deadly Drifters. Jellyfish often cluster together during springtime or when environmental conditions such as an increase in ocean temperature favor it. When conditions are right, jellyfish shift from being a nuisance to being a menace, if accidentally so, for a jellyfish swarm, unlike more aggressive monstrous kin like the giant jellyfish, comprises not aggressive hunters but rather opportunistic strikers.

They do not generally move to attack nearby prey, but their nearly translucent coloration makes it horrifically easy for a creature to swim into a swarm unawares. Once a jellyfish swarm deals damage to a creature, the swarm pursues it for several rounds before giving up the chase. Many aquatic races use jellyfish swarms as defensive guardians, trusting a swarm’s lack of interest in moving to keep it stationary for long periods of time.

Sapphire Stalkers. Sapphire jellyfish are active hunters. They store powerful electrical charges in their domelike bells, which are 16 feet in diameter. Their sensory tentacles can trail for twice that length, but the dangerous toxic tentacles are positioned within about 20 feet of the creature’s bell.

Section 15: Copyright Notice

Sea Monsters (5E) © 2021, Legendary Games; Authors Michael Ritter, Michael “solomani” Mifsud, Robert J. Grady, Mark Hart, Jeff Ibach, Alex Riggs, Scott D. Young, Jeff Lee, Matt Kimmel, and Jason Nelson.

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