Gargantuan monstrosity, chaotic evil

Armor Class: 20

Hit Points: 310hp 15d20+112

Speed: 40 ft , fly: 120 ft

STR

23 +6

DEX

22 +6

CON

22 +6

INT

14 +2

WIS

14 +2

CHA

10 +0

Saving Throws: STR +12, DEX +12, Con +12

Skills: Perception +14, Investigation +14

Damage Resistances: necrotic

Damage Immunities: acid, non.magical bludgeoning, piercing, and slashing, poison

Condition Immunities: petrified, grappled, dieseased, restrained, blinded

Senses: Blindsight 120ft., Tremorsense 300ft., Passive Perception 24, does not have eyes

Languages: Abyssal, Draconic, Infernal

Challenge Rating: 18

Proficiency Bonus: 6

Actions

Multiattack. Bloodwing can use its Frightful Presence. It makes three attacks: one with its bite and two with its bloodshot.   Blood Bite. Melee Weapon Attack: 1d20+12 to hit, reach 10 ft., one target. Hit: 28 4d10+6 piercing damage plus 22 [4d10] acid damage.
Bloodshot. Ranged Weapon Attack: 1d20+12 to hit, range 20/100 ft., one target. Hit: 28 4d10+6 bludgeoning damage plus 13 3d8 acid damage. If the target is a creature, that creature becomes cursed by Bloodwing for 1 Minute. At the end of the targets turn they can make a DC19 Constitution saving throw. While the creature is cursed, Bloodwing has advantage on all its attacks against it.
Blood Breath (Recharge 5-6). Bloodwing exhales acid in a 60-foot line that is 5 ft wide. Each creature in that line must make a DC 20 Dexterity saving throw, taking 54 12d8 acid damage on a failed save, or half as much damage on a successful one.
Frightful Presence. Each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 19 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.

Legendary Actions

Bloodwing 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. Bloodwing regains spent legendary actions at the start of its turn.   Blood Bite. Bloodwing makes a Blood Bite Attack.
Blood Reaper (Cost 2 Actions). All creatures currently cursed by Bloodwing and within 30 feet of it take 15 necrotic damage, which heals Bloodwing for the same amount in total.
Toxic Blood (Cost 3 Actions). Bloodwing fills a 20-foot cube it can is aware of within 120 feet of itself with toxic blood. Each creature in that area must succeed on a DC 18 Constitution saving throw or take 18 4d8 poison damage. If the creature is not at full HP, it becomes inflicted by Rotten Heart Disease

Usual Tactics

With enemies that do not fly, close in to its blindsight range and attack with Bloodshot cursing the enemies before baiting them into close combat. There it focuses on healing itself with Blood Reaper and Biting its targets to death.   It's blood breath is used to disease its enemies and should it not be able to kill it, letting the Rotten Heart disease do it instead.