Geuseunsae 그슨새

Medium undead, neutral evil

Armor Class 13 (incorporeal form)

Hit Points 75 (10d8 + 30)

Speed 0 ft., fly 50 ft. (hover)

STR 6 (−2) · DEX 16 (+3) · CON 16 (+3) · INT 14 (+2) · WIS 17 (+3) · CHA 18 (+4)

Saving Throws Wis +6, Cha +7

Skills Insight +6, Perception +6, Stealth +9

Damage Resistances acid; fire; lightning; thunder; bludgeoning, piercing, and slashing from non-magical attacks

Damage Immunities necrotic, poison

Condition Immunities charmed, exhaustion, grappled, paralyzed, petrified, poisoned, prone, restrained, unconscious

Senses darkvision 60 ft., passive Perception 16

Languages understands Common (Jeju dialect) but communicates only by telepathy 60 ft.

Challenge 5 (1,800 XP)

Traits

Incorporeal Movement. The Geuseunsae can move through creatures and objects as if they were difficult terrain. It takes 5 (1d10) force damage if it ends its turn inside an object.

Daylight Haunt. The spirit is just as potent in bright light. It has advantage on Dexterity (Stealth) checks made in daylight.

Mark of Isolation. As a bonus action, the Geuseunsae chooses one creature it can see within 60 feet (the “marked” target). Until the mark ends, only the marked creature can see or hear the Geuseunsae; it is invisible and inaudible to all other creatures.

The mark ends early if the marked creature starts its turn within 10 feet of a conscious ally who speaks to it. When the mark ends, the Geuseunsae can’t use this trait again until the start of its next turn.

Selective Invisibility. The Geuseunsae is invisible to any creature that isn’t its marked target.

Actions

Tormenting Whispers. Ranged Spell Attack: +7 to hit, range 60 ft., one creature currently marked by the Geuseunsae. Hit: 22 (4d10) psychic damage, and the target must succeed on a DC 15 Wisdom saving throw or use its reaction to make one weapon attack against itself, dealing normal damage. A creature without a suitable weapon instead falls prone and takes 7 (2d6) bludgeoning damage.

Hopeless Wail (Recharge 5–6). The Geuseunsae releases a psychic scream audible only to its marked target. That creature must make a DC 15 Wisdom saving throw, taking 36 (8d8) psychic damage on a failed save, or half as much on a successful one. On a failure the target is also stunned until the end of its next turn.

Reactions

Flicker. When a creature other than its marked target hits the Geuseunsae with an attack, the spirit becomes invisible to that creature until the end of the creature’s next turn.

Weakness

Human Connection. If a conscious humanoid other than the marked target speaks to the marked target within 10 feet, the Geuseunsae has disadvantage on attack rolls and saving throws until the end of its next turn (this is already reflected in the Mark of Isolation trait).

Lore

A ghost of one who died in agony, appearing at night; some say it stretches up to the sky. It stands legs

Geuseunsae (그슨새) is a one-of-a-kind demon found only in the folklore of Jeju Island, Korea. Both its name and its nature are inseparable from the island’s dialect and culture. Geuseun- comes from a Jeju word meaning spoiled, tainted, or wrong, while ‑sae can be read two ways: either as sa, malignant energy/evil spirit, or as a reference to a soul cut down unfairly. Put together, the name can be understood as corrupted spirit, spirit of injustice, or vengeful, tainted ghost.

In the old stories, Geuseunsae has no clear human anatomy. Witnesses describe it as a living straw rain-cloak (jujaeng-i or ujang) that flaps through the air like a pair of wings. The image comes from a grim historical object: on ancient Jeju a woven straw hood was sometimes thrown over a condemned prisoner’s head before execution. Because of that link the creature embodies disgrace, retribution, and violent death. With no face, limbs, or other human features, it inspires a deep, unsettling dread—something that looks as if it should not exist.

From Korean Bestiary 5E, p. 42 · Fern Flower Press