Eventual Comeback. The Far Realm parasite that powers a Thoon infiltrator can survive the death of its host, then revive that host. When a Thoon infiltrator reaches 0 hit points, it doesn’t actually die. All life functions cease, however, and the Thoon infiltrator is functionally a corpse for the purpose of resolving spells that affect dead bodies. It takes a DC 17 Medicine check to detect a slight vibration in the neck-tentacles that indicates that the creature is not fully dead. Removing the tentacles from the neck (a bloody process that requires a whole melee round) kills a Thoon infiltrator for good. If the tentacles aren’t removed, a Thoon infiltrator can effectively resurrect itself. After it has spent a month in the near-death state, a Thoon infiltrator returns to consciousness at full normal hit points. It then frees itself from burial with dimension door and tries to reestablish contact with its mind flayer masters.

Throat Dart. The tentacles that represent a Thoon infiltrator’s parasite aspect can extend themselves up the infiltrator’s throat and out its mouth, from where they can shoot a poison dart at a nearby target. The dart functions as a thrown weapon with a range increment of 10 feet.

Spawn Thrall. If a Thoon infiltrator can spend an uninterrupted minute with an incapacitated humanoid, the infiltrator can turn it into a Thoon thrall by snaking its thin neck-tentacles into the victim’s mouth. Sleeping creatures automatically awaken when a Thoon infiltrator attempts this, so it works only on creatures that are bound, magically held, or truly unconscious. A Thoon infiltrator can create one Thoon thrall per week in this manner.

Submerge Tentacles. A Thoon infiltrator usually hides its neck-tentacles with a cloak, high collar, or scarf. But if it expects close scrutiny, it can submerge the tentacles beneath the flesh, leaving only a rough patch of skin behind. Doing so takes one full round. While its tentacles are submerged, a Thoon infiltrator can’t use its spell-like abilities, throat dart, telepathy, or spawn thrall. If a Thoon infiltrator dies with its tentacles submerged, no search short of dismemberment can reveal the tentacles’ presence, and the Thoon infiltrator’s eventual comeback ability functions normally.

Disguise. The facial features of a Thoon infiltrator are as malleable as clay, granting the creature a +6  bonus on Deception checks.

Innate Spellcasting. The thoon infiltrator's spellcasting ability is Charisma (spell save DC 12). It can innately cast the following spells, requiring no material components:

At will: detect magic

3/day: charm person (DC 14)

1/day: dimension door

Fast Healing: The thoon infiltrator heals 1 hit point per round unless damaged that round with fire, acid or radiant damage.

Throat Dart. Ranged Weapon Attack: +8 to hit, range 10/20 ft., one target. Hit: 2 (1d4+1) piercing damage plus 11 (2d10) poison damage.

Rapier. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 7 (1d6 + 4) piercing damage.