Last Laugh. When Zaresith dies, his amulet flies 40 feet in any direction and summons a beholder

Meta Magic Adept. Zaresith has 3/day charges, he can use one of the charges to use one of the following meta magic options: Transmuted Spell, Extended Spell, Quickened Spell 

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

Spellcasting. The archmage is an 18th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 18, +9 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): fire bolt, light, mage hand, prestidigitation, shocking grasp

1st level (4 slots): detect magic, silvery barbs, mage armor*, magic missile

2nd level (3 slots): scorching ray, mirror image, misty step

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

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

5th level (3 slots): summon draconic spirit, infernal calling, negative energy flood

6th level (1 slot): scatter

7th level (1 slot): reverse gravity

8th level (1 slot): reality break*

9th level (1 slot): time ravage

*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.

Counter spell

Retributive Blast. When a creature deals damage to Zaresith, he can cast fire bolt or chill touch to that creature if the target is within range.