Allisara, Devotee of the Old Ones

Magic Resistance. Allisara has advantage on saving throws against spells and other magical effects.

Spellcasting. Allisara is an 18th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 17, +9 to hit with spell attacks). Shr can cast disguise self and invisibility at will and has the following spells prepared:

Cantrips (at will): eldritch blast, frostbite, [Tooltip Not Found], prestidigitation, shocking grasp

1st level (4 slots): ice knife, earth tremor, mage armor*, witch bolt

2nd level (3 slots): detect thoughts, mirror image, misty step

3rd level (3 slots): counterspell, fly, lightning bolt

4th level (3 slots): banishment, fire shield, stoneskin*

5th level (3 slots): cone of cold, scrying, wall of force

6th level (1 slot): globe of invulnerability

7th level (1 slot): teleport

8th level (1 slot): power word stun*

9th level (1 slot): time stop

*The archmage casts these spells on itself before combat.

Dagger. Melee or Ranged Weapon Attack: +6 to hit, reach 5 ft. or range 20/60 ft., one target. Hit: 4 (1d4 + 2) piercing damage.

Summon minions

Allisara may use her legendary action to summon 1d4 Chuul, 1d6 Intellect Devourers or 1 Mind Flayer.

Eldritch Rebuke

Upon reaching a quarter of her maximum HP, Allisara may use this legendary action to create 1d4 tentacles. These tentacles can be used to multi-attack equal to the number of tendrils summoned, have +9 to hit and do 2d6+4 bludgeoning damage each upon a successful hit.