A host infested by a N'Kai is capable of innate spellcasting, which manifests as four Warlock spells. Their spellcasting ability is charisma (11).
At will: Eldritch Blast
Once per day: Black Tentacles, Blight
Twice per day: Poison Spray
Ambient Magic Absorption: If a magical attack against the host body does more damage than the host HP, the remainder passes on to the N'Kai inside and is absorbed. The N'Kai then exits the host corpse, becoming Hostless, and starts with 11hp, and +1 to each physical stat.
Host Body: When infesting inside of a host, a N'Kai makes changes to the body. The eyes turn an ink-like red, and the host begins to sprout disease like boils.
Aberrant Mind: When being infested, a N'Kai host cannot be charmed and has advantage on saving throws against being frightened.
Multiattack: The N'Kai host casts an Eldritch Blast, and attacks with its claw attack.
Claw: Melee Weapon Attack, +5 to hit. Hit: 1d4 slashing damage. Target must make a DC 10 Strength save, or be grappled. When grappling, the N'Kai host cannot move or take an action against a new target without breaking the grapple.
Bite: Melee Weapon Attack, Grappled targets only. Hit: 1d6 piercing damage. Target must make a DC 12 Constitution save, taking 1d4 poison damage on a failed save. Poison damage will repeat until any healing spell is cast.
Devour: In an emergency, the N'Kai can use a single action to devour the host brain entirely, and forcibly cause the host body to sneeze, riding the harsh air force out of their now dead host. A creature can follow it's movement by succeeding a DC 14 perception check. On a failed save, they cannot find it.
N'Kai are flecks of slime granted a twisted, insane sentience due to contact with a Great Old One. They are corrupted into being by the madness that infests whatever the Old Ones touch, and are left behind like footprints. When inside of a host, the N'Kai coat the host brain and begin slowly devouring it. The host body begins to decompose slowly, almost imperceptibly to untrained eyes at first. The host will begin to go insane, and manifest powers similar to a Warlock shortly before they die and the N'Kai move on to another host.
It is possible for someone to get rid of a N'Kai, but they will need either a powerful Psionic such as a mystic, or a highly skilled healer capable of removing it (Medicine check DC 24). If the host becomes aware of the infesting creature, they must pass a DC 17 WIS save to keep their knowledge hidden from the parasite. If they fail, there is a 25% chance the N'Kai simply devours their brain and moves on, otherwise it will simply begin infecting the host with madness (begin progress on short term madness table) and eventually overtake the host entirely.