Arcturia, Prime Transmuter

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

Spell Sniper. Double range of all spells.

Contingency. When Arcturia falls below 25 hp, he casts 5th level life transference, 6d8, for Halaster. 

Spellcasting. The archmage is a 20th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 21, +13 to hit with spell attacks). The archmage can cast disguise self and invisibility at will and has the following wizard spells prepared:

Cantrips (at will): mind sliver, fire bolt, light, mage hand, prestidigitation, shocking grasp

1st level (4 slots): thunderwave, shield, charm person, magic missile

2nd level (3 slots): vortex warp, mirror image, misty step, hold person*

3rd level (3 slots): counterspell, fireball, dispel magic

4th level (3 slots): banishment*, raulothim's psychic lance, sickening radiance*, polymorph*

5th level (3 slots): steel wind strike, control winds*, synaptic static

6th level (2 slots): globe of invulnerability*, scatter

7th level (2 slots): power word pain

8th level (1 slot): feeblemind

9th level (1 slot): mass polymorph*

Legendary Resistance (3/Day). If Arcturia fails a saving throw, they can choose to succeed instead.

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.

Arcturia has 5 legendary actions. 

Cantrip. Arcturia casts a cantrip.

Allure. Arcturia forces one creature that can see and hear her within 60 feet to make a DC 18 Wisdom save or use its movement on its next turn to approach them. 

Quick Change (2 Actions). Arcturia casts ploymorph on a creature, turning them into a rabbit until the start of its next turn.