Magic Resistance. Ada has advantage on saving throws to resist magical effects. She also has advantage on checks made to maintain concentration

Innate Spellcasting. Ada can cast the following spells with a +12 to hit and a DC of 20. 

At Will. Magic Missile, Detect Magic, Heat Metal

2/day. Scorching Ray, Plane shift, Fear, Destructive Resonance (Void Magic)

Coven Spellcasting. Ada, Estelle, and Myrna can cast spells from this list. They all share the same spell slots, and can only access them while within 50ft of each other. 

1. (4): Identify, Witch Bolt, Shield, Hellish Rebuke
2. (3): Maddening Whispers (Void Magic), Tasha's Mind Whip, Web
3. (3): Counterspell, Lightning Bolt. Haste
4. (3): Nether Weapon (Void Magic), Blight, Dimension Door
5. (2): Living Shadows (Void Magic), Cone of Cold
6. (1): Life Drain (Void Magic)
7. (1): Prismatic Spray
8. (1): Dark Star
9. (1): Blade of Disaster

Staff of the Void. Very Rare. A gnarled black staff with a jagged bit of onyx at its tip. This staff is how Ada taps into the Void's energy. The staff gives Ada a +2 to her Armor Class, Saving Throws, Spell attack rolls, and Spell Save DC's. The Staff has 10 charges which she can spend to cast the following spells through it: Life Drain (6), Void Strike (3), Witch Bolt (1).

• Void-Hunger. Whenever Ada casts a void spell through the Staff of the Void, she gains temporary hit points equal to half the damage dealt by the spell or twice the spells level, whichever is higher. 

Legendary Resistance. The Mirecough sisters each share a pool of 3 legendary resistances.

Claws. +10 to hit, 5ft reach. On hit, 2d8+4 slashing damage

Change Shape. The hag magically polymorphs into a Small or Medium female humanoid, or back into her true form. Her statistics are the same in each form. Any equipment she is wearing or carrying isn't transformed. She reverts to her true form if she dies.

Etherealness. The hag magically enters the Ethereal Plane from the Material Plane, or vice versa. To do so, the hag must have a heartstone in her possession.

Nightmare Haunting (1/day). While on the Ethereal Plane, the hag magically touches a sleeping humanoid on the Material Plane. A protection from evil and good spell cast on the target prevents this contact, as does a magic circle. As long as the contact persists, the target has dreadful visions. If these visions last for at least 1 hour, the target gains no benefit from its rest, and its hit point maximum is reduced by 5 (1d10). If this effect reduces the target's hit point maximum to 0, the target dies, and if the target was evil, its soul is trapped in the hag's soul bag. The reduction to the target's hit point maximum lasts until removed by the Greater Restoration spell or similar magic.

Void Empowered (Recharge 5-6). Ada can infuse a spell she casts with Void magic. If the spell deals damage, Ada can choose to switch the damage type to necrotic. Regardless, the spell deals an additional amount of necrotic damage, 1d12 per the spells level up to a max of 5, and counts as a Void spell. If the spell forces a saving throw, creatures make it with disadvantage.

One-For-All. Each of the sisters is connected. While within 50ft of each other, a sister can use her bonus action to siphon some of their lifeforce through the bond into another sister. They can take an amount of damage in d8 increments up to 18

Legendary Coven. Ada, Myrna, and Estelle's powers combined elevate each of them to legendary status. The Mirecough sisters share the following legendary action table. Additionally, they have 4 Legendary action points.

• Uncanny Swiftness. One of the Mirecough sisters can move up to half their movement without provoking opportunity attacks.
• Vicious Strike. One of the Mirecough sisters can make an attack roll with a weapon, or can spend 2 legendary action points to cast a spell of the 3rd level or lower.
• Soul-Draining Gaze (3). The Sisters all turn their gaze upon one creature they can all see. This creature must make a Charisma saving throw with a 19 DC or take 8d8 necrotic damage and gain a level of exhaustion. The target takes half as much on a success and is not exhausted. The sisters then each gain temporary HP equal to 1/3 of the damage dealt.