Seaweed Siren

Large monstrosity, unaligned

Armor Class 18 (natural armor)
Hit Points 294 (28d10+140)
Speed 30 ft., swim 30 ft.

29 (+9) 16 (+3) 20 (+5) 11 (+0) 16 (+3) 21 (+5)

Saving Throws Str +14, Con +15, Wis +13, Cha +13
Skills Athletics +14, Deception +15, Perception +13, Stealth +13
Damage Resistances fire; bludgeoning, piercing, and slashing from nonmagical attacks
Damage Immunities acid, cold
Condition Immunities charmed, frightened
Senses darkvision 120 ft., passive Perception 23
Languages Aklo
Challenge 17 (18,000 XP)

Special Traits

  • Cacophony. The seaweed siren projects a constant noise that disrupts spellcasting. Any creature casting a spell within 100 feet of the seaweed siren requires a DC 19 Constitution saving throw. Failure means the spell fails to be cast. The action is used, but any spell slots or uses per day are not. In addition, any hearing-based perception checks made within 100 feet of the seaweed siren have disadvantage. The seaweed siren can use its bonus action to stop using this ability, or to reactivate it.
  • Innate Spellcasting. The seaweed siren’s spellcasting ability is Charisma (spell save DC 18). It can innately cast the following spells, requiring no material components:
  • Multi-Headed. While the seaweed siren can see in all directions, and has advantage on Wisdom (Perception) checks, which applies to its passive Perception score. It also has advantage on saving throws against being blinded, deafened, stunned, and knocked unconscious. Whenever the seaweed siren takes 40 or more damage in a single turn, one of its heads dies. If all three heads die, the seaweed siren can no longer use its Cacophony ability, or its innate spellcasting. Heads regrow at a rate of one per day.
  • Water Breathing. The seaweed siren cannot breathe air. It can move on land but must hold its breath.


  • Multiattack. The seaweed siren makes three bite attacks.
  • Bite. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 20 (2d10 + 9) piercing damage and 11 (2d10) poison damage.
  • Sonic Beam (Recharge 4-6). The seaweed siren uses its heads to create a wail that affects everything in a 60-foot line. Each creature in that line must make a DC 18 Constitution saving throw, taking 56 (12d8) thundering damage and becoming deafened on a failed save, or half as much damage and not becoming deafened on a successful one.


Seaweed sirens hunt near the shore, where they wait for clam diggers strolling the beaches, lone fisherfolk, or even passing ships. The creature’s three singing heads sway atop serpentine necks that extend from a bulbous body split by a wide, toothy mouth. Pungent strands of seaweed cover the creature like slimy hair. Once a seaweed siren spots its prey, the creature lurks just under the water and allows its three strange heads to protrude above the surface. These heads are nothing more than appendages, and while it can breathe through them it doesn’t use them to eat. Seaweed sirens’ heads grow differently depending on where the creature developed in order to match the skin tone and apparent ethnicity of the surrounding humanoid population. In addition, the heads are eyeless— the siren sees using the many eyes on its main body mass. A seaweed siren stands over 8 feet tall from the tip of its stubby legs to the top of its heads and is nearly 7 feet in diameter. The creature weighs upward of 3,500 pounds.

Horrific Hybrid. At first glance, this creature appears to blur the line between plant and animal. Three eyeless heads sway above the central body mass, constantly singing, chanting, and speaking in nonsense languages and simple babble. Seaweed covers the creature’s three false heads and its main central body—a form of camouflage to help the beast remain hidden while hunting. Six stout, crablike legs carry this creature along the coast and through the rocky tide pools it inhabits.

Entrancing cacophony. The seaweed siren’s heads sing songs and babble in strange nonsense languages to fuel the seaweed siren’s many special abilities. Even when not in use against a potential meal, the heads seemingly converse with each other, holding lengthy conversations full of random syllables and made-up words. Once it draws its prey near, the seaweed siren attempts to charm or bewilder its foe to gain the advantage. After this, the creature moves closer and begins to devour its still-living victim. While the seaweed siren prefers to dine on living humanoids, it uses its strident squall attacks to incapacitate or kill prey that flees or resists its charm attempts.

Seaweed sirens sometimes ally themselves with other aquatic creatures to share in hunts or for mutual protection. Sahuagin typically don’t trust them but may ally with them long enough to capture new slaves and restock their humanoid food supply. Locathah sometimes use these strange beasts as protectors, keeping them well fed in return for the creatures serving as lookouts and sentinels. Merfolk and aquatic elves avoid seaweed sirens, and even go so far as to sometimes warn other humanoid communities when one is discovered to be hnting nearby.

Seaweed sirens use a form of aggressive mimicry, appearing to have humanoid features in order to lure in their preferred meals. They can converse in Aklo, and constantly babbles in glossolalia, but if it manages to talk with another sentient being that has a language long enough, it begins mimicking the other’s language and speech patterns, eventually sounding exactly like it. Though a seaweed siren can use its tongues spell to understand and speak any language, it prefers to talk with and mimic its conversational partners without resorting to using this ability. Some speculate the creature catalogs every conversation in order to add to the collection of sounds and words that power its cacophony special ability.

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.

This is not the complete section 15 entry - see the full license for this page