Updated Erinyes Assassin

Hellish Weapons. The Erinyes's attacks deal an extra 13 (3d8) Poison damage on a hit(Included in the attack).

Magic Resistance. The Erinyes has advantage on saving throws against magical effects or spells.

Sneak Attack. The Erinyes deals an extra 21 (6d6) damage when it hits a creature, and it has advantage on the roll, or when the target is within 5 feet of an ally of the Erinyes, and the Erinyes doesn't have disadvantage.

Shapechanger. The Erinyes can magically polymorph into a small or medium humanoid of its choice, or into its true form. It reverts to its true form if it dies. Equipment isn’t transformed.

Innate Spellcasting. The Erinyes is a 6th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 17, +9 to hit with spell attacks). The Erinyes has following spells prepared:

At Will: Charm Person, Fire Bolt, Light, Minor Illusion, Thaumaturgy.
3/a Day Each: Silence, Mirror Image, Suggestion.
2/a Day Each: Major Image, Hypnotic Pattern, Charm Monster.

Multiattack: The Erinyes makes two Dagger attacks, or uses any of it's other abilities, and either Dashes, Disengages, or Hides.

Dagger. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 6 (1d4 + 4) Piercing damage, plus 13 (3d8) Poison damage. Additionally, a target must succeed on a DC 17 Constitution saving throw, or become paralyzed for 1 minute. The target can re-roll the saving throw at the end of their turn. 

Charm. A humanoid or Fiend the Erinyes can see within 30 feet must succeed on a DC 17 Wisdom save, or be charmed for 1 day. While charmed, the creature follows the Erinyes’s orders, and can reroll their save if they take damage, or if they are given a suicidal command. If a creature successfully saves, it gains a +2 to all Wisdom saves it makes against this Erinyes’s charm effect until it takes a long rest. It can have an infinite amount of humanoids charmed as long as they are below CR 2. It otherwise can have up to 5 creatures charmed.

Energy Drain. Melee Spell Attack: One creature that is willing, grappled by the Erinyes, Charmed, Restrained, or Incapacitated must succeed on a DC 17 Constitution saving throw, taking 35 (6d10 + 5) Psychic damage on a failure, or half on a success. The Erinyes regains Hit Points equal to half the damage dealt, and the creature’s maximum hit points are reduced by the damage taken. The reduction lasts until the target takes a long rest. If the target’s maximum hit points are reduced to 0, the target dies, and then rises as a Bearded Devil next round, that is friendly to the Erinyes.

Etherealness. As an action, the Erinyes phases into the Ethereal plane from the Material plane, or vice versa.

Right Behind You(2/a Day). When a creature damages the Erinyes, it halves the damage and casts Mislead as a reaction.