Dungeon Swan

The Dungeon Swan lives in dungeons, and it feeds on HATRED. It is notoriously territorial, horrendously violent, and quite beautiful from afar. Get up in its grill and someone's going to lose a liver.

"SWAN!!"
Source

Etymology
The "Dungeon" in Dungeon Swan is a colloquial contemporary tag-on, originating from locals' observations of the Swans' lairs. "Swan", of course, being the harshest of Elven curses - one reserved for mortal calamity, infidelity, and royal gaffs. It is the verbal equivalent of wiping excrement off the floor with silk. Elven scholars widely believe that the creature obtained this name by the reaction the first Elves had to it - cursing in the foulest way they could while presumably running away at speed.

Description
Dungeon Swans are shapeshifters, so any description hence forth should be understood as an impermanent and temporary description of the creature. There do appear to be several favored forms, however:

HORRID PRETTY BIRD: A feathered fowl, reaching at maximum 5 meters in length, weighing around 50 kilograms, composed of either black or white feathers, and hiding a set of wickedly jagged teeth within their bills, of which they use to pierce and rend through the soft places of their prey.

PRIMORDIAL LIZARD: Either a snake with tiny arms or a stumpy lizard without legs. Up to 8 meters in length, with scant-toothed jaws so strong they can bite a cow in half. Scales of purest colors, decorated in hypnotic dancing patterns of black, red, and white.

((Joke: What is black, white, and red all over? Answer: SWAN! FUCK!"))

From Black Swan - 2010

DEADLY JAR: An ornate ceramic jar approximately six feet high with a single S-shaped spout, a handle, and a potted lid. The most dangerous of vase-shaped teapots. Painted in a bright porcelain red, with white and black decor circumventing the body depicting Dungeon Swan eviscerating their victims, or other grisly scenes. The contents of the jar are surmised to consist of pure, distilled hatred, which continuously bubbles and boils, hence the spout to relieve pressure.

AX MURDERER: A war-painted, kabuki-dressed human or elf wielding a bone-shafted obsidian ax. If the Dungeon Swan has recently claimed a victim, then it will also be completely covered in dripping, pulsating entrails. Thought to be capable of speech, but has yet to be witnessed.

Environment
The Dungeon Swan, prefers, of course, the Dungeon - an ecosystem so riddled with continued violence, suffering, and hatred that it can feed, while simultaneously being private enough for it to raise its young. Other popular nesting options include, but are not limited to: torture chambers, gulags, forced labor camps, drow citadels, royal palaces, and office buildings.

It finds aquatic habitats preferable. If the dungeon has some sort of pool or underground lake, the Dungeon Swan, or a pair of mated Dungeon Swans has made nesting within.

Behavior
Dungeon Swans consume Hatred. Hatred grows like grass. In hate-filled places, Hatred is grass: it grows from the dungeon floor, out of the walls, in sandy banks and on hard masoned stone. It creeps up through cracks in the cement until it forms a thick hateful brush - a jungle-mesh of acidic strangling weed. Touching Hatred Grass unprotected will send a person into a full-flown rage, or choke them with asthmatic reeds until they die as purple as Hatred Grass itself. Hatred Grass cannot be burnt, and cutting it releases rage-inducing particles into the air. The only safe way to be rid of it is to eat it. Hence, the delicate and beautiful hate-tango of the Dungeon Swan.

This is what they eat: Pure Hate.*
Source

Solitary: A lone Dungeon Swan will eat up to 15 kilograms of Hatred Grass in a single day. Alone, it will tend to be migratory and adventurous, being so bold as to settle in lesser places of hatred, like the basements of bitter old people or in windmills.

Mated Pair: More dangerous than alone, particularly if they are raising young. A mated pair of Dungeon Swan will typically have settled down in a stably-hated location, and will so for several years. At this point they have staked out their territory, and will ruthlessly attack any and all creatures, big or small, within their lair. Typical Mated Pair behavior involves displays of aggressiveness towards intruders, lurking and plotting, and very angry highly-coordinated dances.

Triumvirate: On rare occasion, the Dungeon Swan will form a triplet. This is the most dangerous of all, as it involves the methodical escalation of mating rituals, resulting in aggressive displays up to 5 miles from the lair. Dungeon Swan in a triumvirate will mercilessly attack anything within sight, causing the surrounding countryside to wither into a blight. Killing one Dungeon Swan in the triumvirate will remedy this situation, although that is not so easy a prospect...

Caution
Aside from the Dungeon Swan being a horrible bastion of spitefulness and violent hate, the very worst thing may be its chief innate defense:

Those that deliver a killing blow to a Dungeon Swan will permanently be consumed by hatred, resulting in immediate indiscriminate berserking behavior and total violent madness.

Symptoms of killing a Dungeon Swan include: spontaneous bruising, internal hemorrhaging, bleeding from any and all orifices, loosening or outright falling-out of teeth, seizures, foaming/bleeding of the mouth, weakness, involuntary screaming, hallucinations about Dungeon Swan, and attacking every other living thing within sight and sound for the rest of ones days.

Source

Symptoms for killing an adolescent Dungeon Swan are less severe, usually resulting in only mutilation and terminal weakness, laced with fits of rage.

Symptoms for killing newborn or young Dungeon Swan are entirely temporary. If it has yet to grow its exterior feathers or scales, then it can safely** killed. Fits of rage typically last for only a minute at most, then symptoms cease.

Hence, it is recommended that one should only attempt the extermination of Dungeon Swan while it is very young, while leaving adult Dungeon Swan the hell alone. Though, it may be worth it for a community to sacrifice someone to deliver the killing blow, then immediate murder them if it means being rid of the Swan for good.

*Fun fact: Hatred Grass can be processed and chewed upon like tobacco to produce Rage in the chewer. This typically lasts for a minute. If it's somehow swallowed, it lasts for days.
**Of course there's no safe way to kill a Dungeon Swan. In a life or death encounter this beast, someone's getting injured and someone's getting killed.