Wrigglegeist (5e Creature)
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Tiny undead, unaligned
Damage Resistances acid, cold, fire, lightning, thunder
Ethereal Sight. The wrigglegeist can see 30 feet into the Ethereal Plane when it is on the Material Plane, and vice versa.
Illumination. The wrigglegeist sheds dim light in a 10 foot radius.
Incorporeal Movement. The wrigglegeist can move through other creatures and objects as if they were difficult terrain.
Innate Spellcasting. The wrigglegeist's innate spellcasting ability is Charisma. The wrigglegeist can innately cast the following spells, requiring no material components:
Etherealness. The wrigglegeist enters the Ethereal Plane from the Material Plane, or vice versa. It is visible on the Material Plane while it is in the Border Ethereal, and vice versa, yet it can't affect or be affected by anything on the other plane.
Parasitic Possession (Recharge 6). One humanoid that the wrigglegeist can see within 5 feet of it must succeed on a DC 12 Charisma saving throw or be possessed by the wrigglegeist; the wrigglegeist then disappears inside the host, and the target is knocked unconscious for 3 (1d4 + 1) rounds. The wrigglegeist can't be targeted by any attack, spell, or other effect, except ones that turn undead, and it takes half of the damage its host takes, after accounting for both creature's immunities and resistances. Additionally, it can encourage or discourage the host from specific courses of action; if the host does no heed these suggestions, it takes 5 (2d4) damage.
The ghost of a long-dead parasite. This small being prefers to feed from within a living organism, phasing deep into the organs before semi-solidifying and melding with surrounding tissue to feed off host nutriets. Generally unnoticed and harmless, it can be driven under some circumstances to more dangerous behavior. A wrigglegeist under normal circumstances lives in a passive equilibrium with its host, and can feed with minimal harm or discomfort for the duration of a host's life span. The creature is unfortunately susceptible to over-stimulation however, and a host intaking too much of a particular nutrient or chemical stimulant can drive a wrigglegeist to aggressive gluttony. A "mad" wrigglegeist may tamper with the nervous system or even damage organs until the host indulges its addiction, and may begin to increase in size until it is capable of fully controlling the host body. When the needs of the ectogeist exceed the capacity of the host's biological systems, it begins to digest host biomass in an attempt to maintain its stability and may expand until it envelops whatever remains of the original host. Depending on the circumstances this entire process from passive parasite to hostile parasitoid may be drawn out over the course of many years or a matter of mere hours, but the end result is always the same: with its host finally annihilated, the bloated wrigglegeist explodes in a destructive blast of heated slime.