Sure, here's a stat block for a powerful undead monster in Dungeons & Dragons fifth edition:

***Ancient Undead Dragon***

*Huge undead, neutral evil*

**Armor Class** 22 (natural armor)

**Hit Points** 666 (36d12 + 324)

**Speed** 40 ft., fly 80 ft.

**STR** 30 (+10) **DEX** 14 (+2) **CON** 28 (+9) **INT** 18 (+4) **WIS** 16 (+3) **CHA** 22 (+6)

**Saving Throws** Dex +11, Con +18, Wis +12, Cha +15

**Damage Immunities** necrotic, poison

**Condition Immunities** charmed, exhaustion, frightened, paralyzed, poisoned

**Senses** blindsight 60 ft., darkvision 120 ft., passive Perception 26

**Languages** understands the languages it knew in life but can't speak

**Challenge** 28 (120,000 XP)

***Undead Fortitude.*** If damage reduces the dragon to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the dragon drops to 1 hit point instead.

***Legendary Resistance (3/day).*** If the dragon fails a saving throw, it can choose to succeed instead.

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

***Actions***

***Multiattack.*** The dragon can use its Frightful Presence. It then makes three attacks: one with its bite and two with its claws.

***Bite.*** Melee Weapon Attack: +19 to hit, reach 15 ft., one target. Hit: 24 (3d10 + 10) piercing damage plus 14 (4d6) necrotic damage.

***Claw.*** Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 20 (3d6 + 10) slashing damage plus 14 (4d6) necrotic damage.

***Tail.*** Melee Weapon Attack: +19 to hit, reach 20 ft., one target. Hit: 22 (3d8 + 10) bludgeoning damage.

***Frightful Presence.*** Each creature of the dragon's choice within 120 feet of it and aware of it must succeed on a DC 23 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the dragon's Frightful Presence for the next 24 hours.

***Breath Weapons (Recharge 5-6).*** The dragon uses one of the following breath weapons.

- ***Death Breath.*** The dragon exhales a blast of necrotic energy in a 90-foot cone. Each creature in that area must make a DC 26 Constitution saving throw, taking 90 (20d8) necrotic damage on a failed save, or half as much damage on a successful one.
- ***Cold Breath.*** The dragon exhales a blast of freezing wind in a 90-foot cone. Each creature in that area must make a DC 26 Constitution saving throw, taking 90 (20d8) cold damage on a failed save, or half as much damage on a successful one.
- ***Fear Breath.*** The dragon exhales a blast of fear-inducing gas in a 90-foot cone. Each creature in that area
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