Vampire-killer Archmage

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 17, +9 to hit with spell attacks). The archmage can cast shield and misty step at will and has the following wizard spells prepared (+3 more):

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

1st level (4 slots): detect magic, disguise self, mage armor*, magic missile

2nd level (3 slots): detect thoughts, mirror image, invisibility, knock

3rd level (3 slots): counterspell, fly, fireball

4th level (3 slots): fire shield, dimension door

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

6th level (1 slot): sunbeam, disintegrate

7th level (1 slot): 

8th level (1 slot): mind blank*

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.