Sources
CR
Type
Size
Alignment
Movement

Archmage

Medium Humanoid (any race), Any alignment

Armor Class 12, 15 with mage armor
Hit Points 99 (18d8 + 18)
Speed 30 ft.
STR
10
+0
DEX
14
+2
CON
12
+1
INT
20
+5
WIS
15
+2
CHA
16
+3
Saving Throws INT +9, WIS +6
Skills Arcana +13, History +13
Damage Resistances damage from spells; nonmagical bludgeoning, piercing, slashing, (from stoneskin)
Senses passive Perception 12
Languages any six languages
Challenge 12 (8,400 XP)

Traits

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

Spellcasting

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 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, identify, mage armor*, magic missile

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): mind blank*

9th level (1 slot): time stop

*The archmage casts these spells on itself before combat.

Actions

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.

Lore

Archmages are powerful (and usually quite old) spellcasters dedicated to the study of the arcane arts. Benevolent ones counsel kings and queens, while evil ones rule as tyrants and pursue lichdom. Those who are neither good nor evil sequester themselves in remote towers to practice their magic without interruption.