Blood Dragon

Legendary Resistance (6/Day). If the Blood Dragon fails a saving throw, it can choose to succeed instead.

Blood Hunter. The Blood Dragon knows the location of all non-undead creatures within 120 ft of it.

Creature of Death. The Dragon bypasses the damage Resistances a creature has to Poison and Necrotic damage. Any creature that is reduced to 0 Hit Points by the Dragon is killed and can only be brought back to life by a Raise the Dead spell cast at level 7. 

Undead Resilience. The Blood Dragon has Advantage on saving throws against spells and other magical effects, as well as effects that turn undead. When the Dragon takes Necrotic damage, it takes no damage and instead regains a number of Hit Points equal to the Necrotic damage dealt.

Undying Monster. If the Blood Dragon is reduced to 0 Hit Points, it current Hit Points are instead reset to 400 Hit Points and it can use the actions in the Mythic Actions section for 1 hr. Award a party an additional 155,000 (XP, 310,000 total) for defeating the Blood Dragon after its Undying Monster activates.

Fly-By. The Dragon doesn't provoke opportunity attacks.

Siege Monster. The Blood Dragon deals double damage to objects and structures.

Multiattack. The Dragon makes three attacks using Rend, Tail, or Crimson Ray in any combination and uses terrifying presence if able.

Rend. Melee Attack Roll: +19 to hit, reach 15 ft. Hit: 32 (5d8 + 10) force damage plus 16 (3d12) Necrotic damage.

Tail. Melee Attack Roll: +19 to hit, reach 25 ft. Hit: 23 (3d8 + 10) force damage plus 21 (4d12) Poison damage and must make a Constitution saving throw, DC 22. Failure: Poisoned for 1 min. While Poisoned, the creature can't regain Hit Points. Success: The creature is Poisoned till the end of its turn.

Crimson Ray. Ranged Attack Roll: +19 to hit, range 120 ft. Hit: 36 (6d12) Necrotic damage.

Terrifying Presence  (recharge 5-6). Every non-undead creature that can see the Dragon must make a Wisdom saving throw, DC 23. Failure: A creature drops whatever it is holding, and has the Frightened condition until it can no longer see the Dragon. When the Dragon is no longer in the creatures line of sight, it can repeat the saving throw at the start of its turn. Success: The creature is immune to this effect for 1 min.

Blood Breath (recharge 5-6). The Dragon vomits blood in a 90 ft cone. Every creature in that area must make a Dexterity saving throw, DC 25. Failure: 72 (12d10) Necrotic damage and be covered in blood. While covered in blood, a creature takes 16 (3d10) Necrotic damage at the end of its turn. A creature or a creature within 5 ft of it can use its action to scrape the blood off. The blood falls off after 1 min. Success: A creature takes half damage only.

Bleeding Night (3/day). Every non undead creature within 80 ft of the Blood Dragon must make a Constitution saving throw, DC 26. Failure: A creature takes 115 (22d12) Necrotic damage. Success: A creature takes half damage only. Failure or Success: The blood dragon regains 100 Hit Points as it drains the life from the creatures around it. Any magical or non-magical plant in this area dies, and plant creatures automatically fails the saving throw.

Hindering Wound. The Dragon makes one attack against a random creature that it can see. If the attack hits, the creature has disadvantage on the next attack roll that it makes.

Wild Thrash. In response to taking damage, the Dragon makes one rend attack against a random creature that it can see.

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

Chase Down (cost 2 actions). The Dragon can move up to its speed and makes one rend or tail attack.

Draining Venom (cost 2 actions). The Dragon makes one Tail attack against one random creature that it can see. If the target fails the saving throw, the target is subjected to the Slow spell til the end of its next turn.

Wings (cost 2 actions). Every creature within 20 ft of the Dragon must make a Strength saving throw, DC 18. Failure: A creature takes 21 (4d12) force damage and is pushed back 30 ft in a straight line from the dragon and has the Prone condition. Success: A creature takes no damage and is pushed back 15 ft.

Mythic Action Uses: 3. If the Blood Dragons Undying Monster trait has activated within the last hour, the Blood Dragon can use the following options as Legendary Actions:

Devouring Jaws (cost 4 actions). The Blood Dragon uses Rend against a random creature that it can see. If the attack hits, the target takes an extra 32 (8d8) Necrotic damage. 

Insight Horror (cost 5 actions). The Blood Dragon uses Terrifying Presence. While a creature has the Frightened condition by this ability, it also has the Paralyzed condition.

Annihilating Moon (1/day). The Blood Dragon uses Bleeding Night. Any creature that fails the saving throw is reduced to 0 Hit Points. This action does not count against the number of uses the dragons has of Bleeding Night.