A Piranha Swarm appears as a churning, chaotic mass of razor-toothed fish, each no larger than a human hand. Their scales glisten with an oily sheen—red, silver, and black flickering as they dart beneath the surface. Tiny crimson eyes burn with an instinctual ferocity, and their gaping maws flash rows of needle-sharp teeth, always gnashing, always tasting.
Behaviour: Hyper-aggressive and relentlessly coordinated, the swarm moves like a single entity—darting, circling, and striking with terrifying precision. The swarm displays an almost supernatural ability to detect weakness: blood in the water, vibrations of struggling prey, or sudden motion. Once engaged, it attacks with frenzied abandon, stripping flesh in seconds and vanishing just as quickly when sated.
Habitat: Piranha Swarms infest slow-moving jungle rivers, sunken ruins, and dark wetlands, especially in tropical or cursed regions. They often lurk in still pools and muddy bends, hiding beneath lily pads or submerged roots, waiting for the first sign of prey. Ancient, abandoned aqueducts and flooded dungeons are also known to house unnaturally large and aggressive swarms.
Modus Operandi: Operating through sensory acuity and pack instinct, the swarm ambushes rather than chases. They lie in wait, drawn to the scent of blood or sudden thrashing in the water. Once a target is located, they explode into motion—overwhelming it with dozens of simultaneous bites. They don’t aim to disable or subdue—they simply eat, tearing their target apart in seconds. Larger creatures are devoured alive unless they escape the water swiftly.
Motivation: Driven purely by hunger and instinct, the Piranha Swarm is not malicious—it is nature’s purest expression of consumption. Some scholars argue swarms are drawn to negative energy or sites of long-standing violence, possibly explaining their unnatural aggression in certain regions. To a swarm, there is no fear, no thought—only the drive to feed, shred, and survive.
Swarm. The swarm can occupy another creature’s space and vice versa, and it can move through any opening large enough for a Tiny creature. The swarm can’t regain hit points or gain temporary hit points.
Blood Frenzy. The Piranha Swarm has advantage on melee attack rolls against any creature that doesn’t have all its hit points.
Aquatic Ambush. While motionless underwater, the swarm has advantage on Dexterity (Stealth) checks made to hide. If it surprises a creature, that creature takes an additional 7 (2d6) piercing damage from the first successful attack.
Scent of Blood. The swarm can detect bleeding creatures (those missing any hit points) within 60 feet, functioning as blindsight for this purpose.
Actions
Swarming Bites.Melee Weapon Attack: +5 to hit, reach 0 ft., one creature in the swarm’s space. Hit: 21 (6d6) piercing damage, or 10 (3d6) if the swarm has half its hit points or fewer.
Feeding Frenzy (Recharge 5–6). The swarm erupts in a violent flurry. Each creature within 5 feet that is missing any hit points must succeed on a DC 13 Dexterity saving throw or take 14 (4d6) piercing damage and be grappled (escape DC 12) as dozens of piranhas latch on. A grappled creature is restrained, and the swarm cannot use Swarming Bites on another target until the grapple ends.
Tactics
Ambush Predator: The swarm lies in wait in murky water, launching surprise attacks with Aquatic Ambush for maximum damage.
Target the Wounded: With Scent of Blood and Blood Frenzy, it prioritizes bleeding targets and pursues them relentlessly.
Devour and Scatter: In danger, it uses Feeding Frenzy to disable prey and retreat or reposition.
Environmental Advantage: Most effective in stagnant pools, flooded ruins, or concealed channels where visibility is low and escape is difficult.
This school of red-bellied fish swims swiftly, each tiny mouth filled with rows of razor-sharp teeth ready to feast on flesh.
Piranha Swarm CR 3
XP 800 N Tiny animal (aquatic, swarm) Init +7; Senses blindsense 30 ft., keen scent, low-light vision; Perception +5
DEFENSE
AC 15, touch 15, flat-footed 12 (+3 Dex, +2 size) hp 26 (4d8+8) Fort +6, Ref +7, Will +2 Defensive Abilities swarm traits
OFFENSE
Speed swim 30 ft. Melee swarm (1d6 plus bleed and distraction) Space 10 ft.; Reach 0 ft. Special Attacks bleed (1d6), distraction (DC 14), feeding frenzy
STATISTICS
Str 3, Dex 16, Con 15, Int 1, Wis 12, Cha 2 Base Atk +3; CMB —; CMD — Feats Improved Initiative, Skill Focus (Stealth) Skills Perception +5, Stealth +19, Swim +15
SPECIAL ABILITIES
Feeding Frenzy (Ex)
Against a foe taking bleed damage, a piranha swarm deals 2d6 points of damage with its swarm attack rather than 1d6 points of damage.
ECOLOGY
Environment warm freshwater Organization solitary, shoal (2–5 swarms), or school (6–10 swarms) Treasure none
Shoals of piranhas are voracious feeders, capable of stripping the flesh from the bones of large animals with astonishing speed. These fish have garnered a well-earned reputation in regions where they thrive, but while they are native to tropical rivers, many species of piranha are incredibly hearty. Collectors of exotic pets sometimes ship tanks of these carnivorous fish home, and some unscrupulous owners even put the fish to work disposing of unwanted bodies. When the authorities draw near, panicked criminals sometimes dump the fish into nearby rivers, inadvertently unleashing schools in waters woefully unprepared for their ravenous
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