Cailleach

Overflowing Power. Whenever Cailleach casts a spell, her abundance of power allows her to attach additional spells to the casting. She casts False Life and a Cantrip of her choice (damage adjusted as if she were casting it as an 11th level caster) as well in that order. 

Spellcasting. Cailleach is a 20th-level spellcaster. Her spellcasting ability is Intelligence (spell save DC 25, +17 to hit with spell attacks). Cailleach has the following wizard class spells prepared:
Cantrips (at will): Toll the Dead, Sapping Sting, Chill Touch 

At will spells: Shield, False Life, Fly, Absorb Elements, Counterspell, Vampiric Touch, Inflict Wounds, Dispel Magic

3/day : Teleport, Telekinesis. Harm, Circle of Death, Cloudkill, Legend Lore, Scrying

1/day : Gate, Time Ravage, Abi-Dalzim's Horrid Wilting, Finger of Death, Project Image, Reality Break

Incorporeal Movement. Cailleach can move through other creatures and objects as if they were difficult terrain. She takes (5d10) force damage if she ends her turn inside an object.

Ethereal Sight/Touch. Cailleach can see 60 ft. into the Ethereal Plane when she is on The Material Plane, and vice versa.  She can also target objects in either Plane regardless of the Plane she is in.

Legendary Resistance (5/Day). If Cailleach fails a saving throw, she can choose to succeed instead.

Death Lordship.  Any undead creature, within 60 feet, regardless of whether they can hear her, must pass a DC 25 Wisdom saving throw or obey her commands, save ends (she commands with her free action).

Limited Foresight.  Once per round, Cailleach can make an attack roll against her come at disadvantage (or without advantage if it had advantage) or make a saving throw with advantage (or make it without disadvantage if it had disadvantage).

Unstoppable Spell Caster.  Spells or effects that would prevent a spell from happening, such as a Beholder's Anti-Magic Cone, Silence, or AntiMagic Field, do not stop Cailleach from casting but she takes 10 irreducible damage that round to cast spells.  Spells in the Anti-Magic Cone or Antimagic Field disappear at the end of the round.  

Godly Spell Caster.  Any damage dice on a spell that are a 1 or 2 count instead as a 3.

Staff. Melee Weapon Attack: +9 to hit, reach 5 ft., (1d8 + 3) Magical Bludgeoning damage and (3d6) necrotic damage. 

Counterspell. Cailleach casts counterspell at 5th level and its reach is 150 feet.

Cailleach can take 5 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. Cailleach regains spent legendary actions at the start of her turn.

Disruption of the Weave (Costs 3 Actions). Any enemies within 120 feet of her must make a concentration saving throw, DC 10, with a -4 penalty for any spells they are concentrating on.

Deadly Touch (Costs 2 Actions).  Cast Inflict Wounds with a critical hit range of 17-20.  

Move (Costs 2 Actions). Cailleach moves up to her speed without provoking opportunity attacks.

Godly Competence (Costs 1 Action).  Roll with Advantage on your next attack roll.

Detect (Costs 1 Action). Cailleach makes a Wisdom (Perception) check with advantage.

Extreme Body and Mind (Costs 4 Actions).  Regain a used Reaction.  This means she can take another Reaction.