I built this for Curse of Strahd some time back but never got to use it as my PCs voted not to play Curse.  I dug this up as my PCs as part of my Clash of the Titans campaign when they went to visit the Blood King as a Halloween adventure, and, of course, ended up in a boss fight.  The party had an average level of 16 for reference.  This “vampire king” could be used for any 5e vampire boss fight. In my game the vampire king was Mayong Mistmoore.

Explanatory Notes:

• Phase II activates when phase I is reduced to 0 hit points.
• No effects, conditions or damage carry over from phase I to phase II.
• Saving Throws and Ability Scores are abbreviated as one stat line.
• The AC of the Blood King determines ALL DCs for his abilities.  This should be made known to the PCs at the start of the fight. This is known as the Target DC.
• A bane means the roll has -1d6.
• A boon means the roll has +1d6.
• I ask the PCs to roll a 1d4 as the timer for each encounter. This clocks down by 1 at the start of the Blood King’s turn.  When it hits 1 something happens (as outlined in the timer section). I listed what I used but I run a more gonzo game so feel free to replace it. The key issue is the timer effects should be annoying but not deadly and easy to run for the DM (so 1 attack, no reactions, rounded HPs etc).
• Though I included Lair Actions, I never used them.  Was plenty going on already.

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MAYONG MISTMOORE

The Blood King

Phase I – Handsome Dark Elf

AC 18 (gothic plate), HP 300; Speed 30 ft.
STR +4; DEX +9, CON +4, INT +3, WIS +7, CHA +9; +5
DR necrotic; bludgeoning, piercing, and slashing from nonmagical attacks
Immunities poison; poisoned
Senses Darkvision 120 ft., PP 17; AL LE; CR 15 (13,000 XP)

• Dramatic Entrance. The Blood King always goes first at the start of any encounter (first round of combat).
• Energy Drain. A creature hit by the Blood King’s bite attack suffers 1 level of “energy drain”. This is a counter against the creature’s level. If the creature’s level is reduced to zero they die. It takes 1 long rest or 1 greater restoration to reverse 1 level of energy drain.
• Shapechange. bat, cloud of mist, or back into his true form.
• Legendary Resistance (x 3).
• Regeneration 20.
• Misty Escape.
• Spider Climb.
• Vampire Weaknesses.

Actions (x1)

• Rebi Ra! Mayong uses his obsidian blade to attack each target with a blast of radiant energy.  All creatures of The Blood King’s choosing within 120 feet he can see must make a DEX saving throw or be hit by the obsidian blade attack. This attack also shatters force effects on contact.

Reactions (x1)

• Living Blood. When bloodied (1/2 hp) the vampire creates a living blood (Blobulord) creature that acts immediately. This does not count as a reaction but otherwise follows the same rules.
• Discoporation of Demon Bats. When hit by an attack Mayong may nullify the attack as a reaction. He turns into a swarm of demonic bats and reforms up to 60 feet away.

Legendary Actions (x3 or Party Size/2 – whichever is higher)

• Obsidian Blade. Melee Weapon Attack: +9 to hit, reach 5 ft., one creature. Hit: 11 (2d6 + 4) slashing damage. Creatures hit by this attack gain two stacks of burn damage.  At the start of its turn, it takes 10 (3d6) radiant damage and the blood king is healed for the same amount. These stacks are cumulative but only trigger 1 at a time.
• Enervating Slam. Melee Weapon Attack: +9 to hit, reach 5 ft., one creature. Hit: 8 (1d8 + 4) bludgeoning or piercing damage. Instead of dealing damage, the vampire can grapple the target (escape DC 18).
• Draining Bite Melee Weapon Attack: +9 to hit, reach 5 ft., one willing creature, or a creature that is grappled by the vampire, incapacitated, or restrained. Hit: 7 (1d6 + 4) piercing damage plus 10 (3d6) necrotic damage. The target’s hit point maximum is reduced by an amount equal to the necrotic damage taken, and the vampire regains hit points equal to that amount. The reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0. A humanoid slain in this way and then buried in the ground rises the following night as a vampire spawn under the vampire’s control.
• Dominating Gaze. Save or be dominated for 1 minute, only 1 creature at a time. Save ends, but the target then has -2 to all saves while in the presence of the Blood King.
• Children of the Night. The vampire magically calls 2d4 swarms of bats or rats, provided that the sun isn’t up. While outdoors, the vampire can call 3d6 wolves The called creatures arrive in 1d4rounds, acting as allies of the vampire and obeying its spoken commands. The beasts remain for 1 hour, until the vampire dies, or until the vampire dismisses them as a bonus action.

Timer (1d4)

Doomflayer! (Rats on Motorcycles). Three rats join the battle using warpfire spear attacks that do radiation damage and can trigger mutations.  Another 3 join each time the timer runs out.  They flee when phase 2 starts. I just used a basic stat block for these guys:

DOOMFLAYER (RATS ON MOTORCYCLES)
AC 18 (warpstone leather); HP 100 for each group of 3; Speed 30 ft.; Saves +4; CR 12 as a group of 3 (8,400 XP)
Keen Smell. The skaven has advantage on Wisdom (Perception) checks that rely on smell.
Doomflayer. If the skaven isn’t mounted, it can use a bonus action to remount its doomflayer (motorcycle). While mounted and not incapacitated, the skaven gains the following benefit:

• It can’t be surprised, and both it and its mount have advantage on Dexterity saving throws.
• It has a movement speed of 120 feet.
• The skaven does not provoke attacks of opportunity.
• If the skaven is reduced to 0 hit points while riding its mount, the mount is reduced to 0 hit points as well.

Actions (x3)
Warpfire Spear. Range Weapon Attack: +9 to hit, range 50 ft., one target. Hit: 12 (2d6 + 5) piercing damage plus 10 (3d6) radiation damage.

Inspired by the Groundlings in episode 1 of Thundarr The Barbarian.

BLOBULORD (LIVING BLOOD, As a reaction when Bloodied)
AC 18; HP 100; Speed 20 ft. (bounce); Saves +4
Immunities Blinded, Charmed, Deafened, Exhaustion, Prone
Senses Blindsight 60 ft. (blind beyond this distance), PP 15; CR 10 (5,900 XP)
Amorphous. The oblex can move through a space as narrow as 1 inch wide without squeezing.
Aversion to Fire. If the oblex takes fire damage, it has disadvantage on attack rolls and ability checks until the end of its next turn.

Actions

• Bubble Stream. Melee Weapon Attack: +7 to hit, range 50 ft., one target. Hit: 17 (4d6 + 3) bludgeoning damage plus 7 (2d6) psychic damage. 
• Eat Memories (On a natural 18+). Blobulord targets one creature it can see within 5 feet of it. The target must succeed on a WIS saving throw or take 44 (8d10) psychic damage as blobulord licks it and become memory drained until it finishes a short or long rest or until it benefits from the greater restoration or heal. Constructs, oozes, plants, and undead succeed on the save automatically. While memory drained, the target must roll a d4 and subtract the number rolled from any ability check or attack roll it makes. Each time the target is memory drained beyond the first, the die size increases by one: the d4 becomes a d6, the d6 becomes a d8, and so on until the die becomes a d20, at which point the target becomes unconscious for 1 hour. The effect then ends. When an oblex causes a target to become memory drained, the oblex learns all the languages the target knows and gains all its proficiencies, except any saving throw proficiencies.

 

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MAYONG MISTMOORE

The Blood King

Phase II – Monstrous Man-Bat (Huge, Image)

AC 14, HP 600; Speed 40 ft., fly 40 ft.
STR +6; DEX +9, CON +13, INT +3, WIS +7, CHA +9; +6
DR acid, cold, fire, lightning, thunder; bludgeoning, piercing, and slashing from nonmagical attacks
Immunities necrotic, poison; exhaustion, frightened, grappled, paralyzed, petrified, poisoned, prone, restrained

Senses  Blindsight 60 ft., Darkvision 120 ft., PP 17; AL LE; CR 20 (25,000 XP)

• Legendary Resistance (3/Day).
• Misty Escape.
• Regeneration 20.
• Echolocation. The Blood King can’t use its blindsight while deafened.
• Keen Hearing.The Blood King has advantage on Wisdom (Perception) checks that rely on hearing.
• Annihilating Aura. Any creature that starts its turn within 30 feet of the Blood King take 1d6 necrotic damage and grant the Blood King advantage on attack rolls against it until the start of the creature’s next turn. Undead are immune to this aura.
• Life Eater. A creature reduced to 0 hit points from damage dealt by the Blood King dies and can’t be revived by any means short of a wish
• Energy Drain. All necrotic damage done by the Blood King reduces the target’s maximum hit point total until they take a long rest. The target dies if this effect reduces its hit point maximum to 0. A humanoid slain in this way and then buried in the ground rises the following night as a vampire spawn under the vampire’s control.
• Overpowering Presence. Creatures have a bane on saving throws against the Blood King’s saving throw DCs.

Actions x2

• Multiattack. The Blood King uses Enervating Focus twice, or it uses Enervating Focus and Finger of Doom, if available.
• Bite. Melee Weapon Attack: +12 to hit, reach 10 ft., one incapacitated, or restrained Hit: 7 (1d6 + 4) piercing damage plus 10 (3d6) necrotic damage and the Blood King regains hit points equal to that amount. 
• Enervating Slam. Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: 28 (5d8 + 6) necrotic damage. 
• Finger of Doom (1/Turn).The Blood King points at one creature he can see within 300 feet of it. The target must succeed on a WIS saving throw or take 26 (4d12) necrotic damage and become frightened until the end of the nightwalker’s next turn. While frightened in this way, the creature is also paralyzed. If a target’s saving throw is successful, the timer automatically resets to 1.

Reaction (x1)

Suck. The. Blood. If a creature becomes incapacitated or restrained within 10 ft. of the vampire, he may use his bite attack on the target.

Legendary Actions (x3 or Party Size/2 – whichever is higher)

• Move and/or any available action except for multiattack.

Timer (1d4)

Doom! Add a saving throw and to-hit bane each time the timer runs down. 

Lair Actions (x1, roll 1d4, Optional)

1. Pass through solid walls, doors, ceiling and floors.
2. Locks/Opens as many doors as he likes.
3. Summons an angry spirit of a devoured humanoid – spectre.
4. The target’s shadow animates and attacks as a shadow on a failed CHA saving throw. Disadvantage while detached. Remove curse fixes it after the shadow is destroyed.