Ancient Black Dracolich

Legendary Resistance (4/Day, or 5/Day in Lair). If the dracolich fails a saving throw, it can choose to succeed instead.

Life Suppression. Creatures within 60 feet of the dracolich can’t regain Hit Points.

Magic Resistance. The dracolich has Advantage on saving throws against spells and other magical effects.

Soul Gem. The dracolich has a magical gem. If the dracolich is destroyed while the gem is on the same plane of existence as it, the dracolich gains a new body in 1d20 days, regaining all its Hit Points and appearing within 5 feet of the gem.

The gem is a Tiny object that has AC 20; HP 50; and Immunity to Necrotic, Poison, and Psychic damage. The gem regains all its Hit Points at the end of every turn, but it turns to dust if reduced to 0 Hit Points. If the gem is destroyed, the dracolich can create a new one by completing an 8-hour ritual using a gem worth 1,000+ GP and by expending 5,000 GP, which the ritual consumes.

Multiattack. The dracolich makes three Rend attacks. It can replace one attack with a use of Spellcasting to cast (A) Fear, (B) Melf’s Acid Arrow (level 4 version), or (C) Ray of Sickness (level 3 version).

Rend. Melee Attack Roll: +15, reach 15 ft. Hit: 19 (2d10 + 8) Slashing damage plus 6 (1d12) Acid or Necrotic damage (dracolich's choice).

Breath Weapon (Recharge 5–6). The dracolich uses one of the following breath weapons:

Acid Breath. Dexterity Saving Throw: DC 22, each creature in a 90-foot-long, 10-foot-wide Line. Failure: 72 (16d8) Acid damage. Success: Half damage.
Necrotic Breath. Constitution Saving Throw: DC 22, each creature in a 90-foot Cone. Failure: 78 (12d12) Necrotic damage. Success: Half damage.

Spellcasting. The dracolich casts one of the following spells, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 21, +13 to hit with spell attacks):

At Will: Detect Magic, Fear, Melf’s Acid Arrow (level 4 version), Ray of Sickness (level 3 version)
1/Day Each: Create Undead (level 8 version), Finger of Death, Speak with Dead, Vitriolic Sphere (level 5 version)

Legendary Action Uses: 3 (4 in Lair). Immediately after another creature’s turn, the dracolich can expend a use to take one of the following actions. The dracolich regains all expended uses at the start of each of its turns.

Cloud of Insects. Dexterity Saving Throw: DC 21, one creature the dracolich can see within 120 feet. Failure: 33 (6d10) Poison damage, and the target has Disadvantage on saving throws to maintain Concentration until the end of its next turn. Failure or Success: The dracolich can’t take this action again until the start of its next turn.

Pounce. The dracolich moves up to half its Speed, and it makes one Rend attack.

Terrifying Presence. Wisdom Saving Throw: DC 21, each creature in a 30-foot Emanation originating from the dracolich. Failure: 11 (2d10) Psychic damage, and the target has the Frightened condition until the end of its next turn. Failure or Success: The dracolich can’t take this action again until the start of its next turn.