Environment any aquatic
Organization solitary or bloom (2–8)
Treasure none

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.