Coven Magic. While at least two allied hags are within 30 feet of it, the hag can cast Augury, Find Familiar, Identify, Locate Object, Scrying or Unseen Servant, with no Material components, the spell's normal casting time, Intelligence as its spellcasting ability and a spell save DC of 14. After casting a spell this way, it can't cast that same spell with this trait again until it finishes a Long Rest.
Magic Resistance. The hag has Advantage on saving throws against spells and other magical effects.
Soul Bag. The hag owns a soul bag, and it can use Nightmare Haunting only while it holds or carries the bag. The bag has AC 15, 20 Hit Points and Resistance to all damage. At 0 Hit Points the bag crumbles to dust and frees every soul inside it. The hag can make a replacement bag after 7 days.
Actions
Multiattack. Makes two Claw attacks.
Claw. Rends a target with iron-hard claws.
Nightmare Haunting. Requires the soul bag; once per day. While on the Ethereal Plane, the hag casts Dream (Intelligence, spell save DC 14). The hag must be the spell's messenger, and the target must be a creature on the Material Plane that the hag can see. The spell fails and is wasted if the target is protected by Protection from Evil and Good or stands within a Magic Circle. Damage the target takes from the spell also lowers its Hit Point maximum by the same amount. If the spell kills the target, its soul is trapped in the soul bag, and the creature can't be raised from the dead until the soul is freed.
Spellcasting. The hag casts one of these spells, needing no Material components and using Intelligence as its spellcasting ability (spell save DC 14). At will: Detect Magic, Etherealness, Magic Missile (cast as a level 4 spell). 2/day each: Phantasmal Killer, Plane Shift (self only).
Bonus actions
Shape-Shift. The hag turns into a Small or Medium Humanoid, or back into its true form. Its statistics stay the same in every form except its size. Equipment it wears or carries doesn't change with it.