Magic Resistance. Teremini has Advantage on saving throws against spells and other magical effects.

Time Stop. 1/Day: Teremini stops time for 1d4+1 rounds. She will first cast Wall of Force to trap martials and Crown of Stars. Then Mirror Image, Blink, and/or Mind Blank. On her last turn available she will cast Cone of Cold or another high damage spell that makes sense for the situation. 

Multiattack. Teremini makes four Moon Burst attacks.

Moon Burst. Melee or Ranged Attack Roll: +9, reach 5 ft. or range 150 ft. Hit: 27 (4d12) Radiant damage.

Spellcasting. Teremini casts one of the following spells, using Intelligence as the spellcasting ability (spell save DC 17):

At Will: Blink, Detect Magic, Detect Thoughts, Feather Fall, Light, Mage Armor (included in AC), Mage Hand, Prestidigitation
2/Day Each: Fly, Lightning Bolt (level 7 version)
1/Day Each: Cone of Cold (level 7 version), Sunburst, Scrying, Teleport

Crown of Stars (At Will). Seven motes of moonlight orbit Teremini’s head. She can use a bonus action to send one of the motes streaking toward one creature or object within 120 feet of her. When she does so, make a ranged spell attack. On a hit, the target takes 4d12 radiant damage.

Protective Magic (3/Day). Teremini casts Counterspell or Shield or Feather Fall in response to the spell’s trigger. 

Teremini makes a Crown of Stars attack after each PCs turn if available. 

Legendary Resistance. If Teremini fails a saving throw she can choose to succeed. However, each time she does so, she loses one use of Crown of Stars. So each time she uses a legendary resistance she loses a legendary action.