Poisonous Strikes. The weapons held by the hollow skin are coated with poison. Its attacks deal extra poison damage included in the actions.
Hide Presence. The hollow skin's creature type appears like whatever it wants to be when targeted by a feature, spell, or magical effect that would affect it, for example a paladin's divine sense ability or the detect evil and good spell.
Mark for Destruction (1/Day). The hollow skin chooses a damage type and touches a creature to force it to make a DC 12 Wisdom saving throw. On a failed save, the next time it is hit with the damage type of the hollow skin's choice, it is counted as vulnerable to the damage type. If the target has immunity to the chosen damage type, this feature does not work. On a successful save, the next time it is hit with the damage type of the hollow skin's choice, it takes an extra 7 (2d6) necrotic damage and the mark ends. While the mark continues, the hollow skin knows where the target is for 1 hour, unless it is on another plane of existence.
Actions
Dagger. Melee or Ranged Weapon Attack: +3 to hit, reach 5 ft. or range 20/60 ft., one target. Hit: 3 (1d4 + 1) piercing damage plus 4 (1d6) poison damage.
Unnoticed
These fiends are almost-human. Their flesh, body, eyes, mind, and everything is the same as a human. They are in their own body, no fiend possessed them, but they cease to be humans anymore. They lost their souls, gave up on their breath, or some other vile deed transformed what they are into mere fiends.
Silently Dangerous
They are not worse than a human with evil intentions, but they hide what they truly are and await their time for malice. Even adept spellcasters cannot easily identify these fiends. Thus, they can be anywhere, hiding in plain sight—smiling at your face or shaking your hand.
Mark for Greater Evil
Their most dangerous threat is to leave a mark for a bigger and more dangerous hunter. They silently and subtly mark a creature they desire, and the marked ones usually become the targets of more dangerous devils, evildoers, and deceivers.