Archnecromancer

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

Spellcasting. The Archnecromancer is an 18th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 21, +13 to hit with spell attacks). The Archnecromancer can cast disguise self and invisibility at will and has the following wizard spells prepared:

Cantrips (at will): fire bolt (4d10), light, mage hand, prestidigitation, chill touch (4d8)

1st level (4 slots): detect magic, false life, mage armor, magic missile

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

3rd level (3 slots): counterspell, fly, animate dead

4th level (3 slots): banishment, blight, stoneskin

5th level (3 slots): cone of cold, dream, creation

6th level (1 slot): create undead

7th level (1 slot): teleport

8th level (1 slot): clone

9th level (1 slot): weird

The Archnecromancer casts mage armoron itself before combat. It may also have false life and/or stoneskin cast if it has warning soon before combat, and it may have animate dead cast. (The CR assumes no created undead).

Undead Thralls. Undead created by you have there hit point maximum increased by 18, and get +6 to there weapon damage rolls.

Dagger. Melee or Ranged Weapon Attack: +9 to hit, reach 5 ft. or range 20/60 ft., one target. Hit: 4 (1d4 + 3) piercing damage.

Command Undead. Choose one undead you can see within 60 ft. of you. It makes a DC 21 Charisma save. On a failure, it becomes friendly to you, and obeys your commands. On a success, it is immune to your Command Undead action permanently. If it has an Intelligence of 8 or higher, it has advantage on the save. If it has an Intelligence of 12 or higher, it gets to remake the save at the end of each hour, ending the effect on a success.

The Archnecromancer can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The Archnecromancer regains spent legendary actions at the start of its turn.

Cantrip. The Archnecromancer casts a cantrip of its choice.

Spell (Costs 2 actions). The Archnecromancer casts a spell of no more than 5th level of its choice.

Control Undead (Costs 3 actions). Each undead creature under the Archnecromancer's control takes an action.