Blight

Amphibious. Blight can breathe underwater and on land. 

Many heads. If Blight takes 20 damage or more on his turn he loses one head. Blight usually has eight heads. At the start of his turn Blight regains two heads for every headiest lost in this way he begins 10 hit points for every two heads regrown. This phrase is temporarily suppressed if he has taken radiant damage on his last turn.

Commanding presents. Any undead or demon within 100 ft of Blight must succeed on a 30 wisdom saving throw or must serve Blight till Blight or the creature being commanded dies.

Diabolical Resurrection. When flight is killed his body is reduced to bad smelling mist and he reforms in the 2689 layer of the abyss over the course of 1D10 years blight cannot escape the abyss again for 10d100 years. If he is killed in the 2689 layer of the abyss he cannot reform for 1D100 years he cannot escape the best for 33d100 years.

Demonic regeneration. At the start of lights turn if he is bloodied he can regenerate 5 HP. This trait is suppressed if he takes radiant damage on his last turn.

Spell master. Blight can concentrate on multiple spells at once and still attack well concentrating on spells. He also does not need spell components to cast spells.

Acidic rage. This trait is activated when Blight drops to 0 HP.  Blight games access to mythical actions. Regenerates to 444 health(222 HP is now blooded). If he's defeated after this give the players an extra 150,000 XP.

Spellcasting. Its spellcasting ability is Wisdom, spell save DC 30, +15 to hit with spell attacks. The blight has following spells prepared:
at will: Acid Splash, animate dead. Antilife shell, armor of Agathys, arms of hadar, bane, Banishment, bestow curse, blade ward, blight(15d8 necrotic damage), blindness/deafness, charm monster, charm person, chill touch, chromatic orb, comprehend languages, control water, counterspell, creat or distoroy water, crown of madness, darkness, dominate beast, evard's black tentacles, false life, hail of thorns, hex, hold person, hunger of hadar, inflict wonders, Melf's acid arrow, phantasmal killer, poison spray, ray of enfeeblement, ray of sickness, silence, sleep, speak with dead, vampiric touch, vitriolic sphere.
40 times a day: Antimagic feild, befuddlement, blade barrier, circle of death, cloud kill, contagion, control weather, create undead, destructive wave, Disintegrate, dominate monster, dominate person, eye bite, finger of death, flesh to stone, harm, hold monster, insect plague, power word stun, power word kill, prismatic spray, prismatic wall, storm of vengeance, sommon fiend(demon only), sommon undead.                       2 times a day: stop time, wish.

Multiattack. Blight makes 4 claw, 2 tale wack, and 1 bite attacks or 2 claw, 1 tale wack, 1 poison shot, and 1 acid breath.

Bite. Melee Weapon Attack:  reach 25 ft, DC 23 dexterity. Hit: 27 (2d10 + 7) pieceing damage and is grappled. Bight can have up to eight targets grappled in this way.

Claw. Melee Weapon Attack: +5 to hit, reach 20 ft. Hit: 25 (5d5) Slashing damage.

Tale wack. Melee Weapon Attack: reach 25 ft, DC 20 dexterity. Hit: 49 (4d10 + 9) bludgeonin damage.

Poison shot. Ranged Weapon Attack: +16 to hit, range 30/100 ft. Hit: 37 (3d10 + 7) poison damage.

Acid breath. Ranged Weapon Attack: range 30/100 ft, hit DC 17 dexterity, 25 ft cone. Hit: 37 (3d10 + 7) acid damage. Miss half damage. 

Many headed acid breath(recharge 6). Ranged Weapon Attack: range 30/100 ft, hit DC 28 dexterity, 200 ft cone. Hit: 296 (24d10 + 56) acid damage. Miss half damage.

Tongue. Blight attacks one of the creatures he has grappled from the bite attack. Melee Weapon Attack: reach 5 ft, DC 23 dexterity. Hit: 29 (2d10 + 9) bludgeonin damage.

Swallow. Blight attacks one of the creatures he has grappled by the bite attack. Hit 28 dexterity saving throw. Hit the player gets the incapacitated condition, 34 (3d10 +4) acid damage on each of blight's turns(no action required). If blight takes 20 or more damage on one of his turn he regurgitates one creature within 20 ft of him this creature lands prone. 

Reactive acid. Blight makes this attack after he is hit with a melee attack causing a pool of acid to spring out of the ground underneath of Bights attacker. Melee Weapon Attack: reach 15 ft., DC 15 dexterity target. Hit: 15 (1d8 + 7) acid damage.

Poison spit. Blight makes this attack after he is hit with a ranged attack.  Ranged Weapon Attack: +5 to hit, range 20/80 ft. Hit: 9 (1d10 - 1) Poison damage.

After a character takes there turn Blight can use a legendary action if he did not use a reaction.

Wing attack. This attack hits anyone in a 15 ft radius. Melee Weapon Attack: 15 target. Hit: 23 (2d10 + 3) bludgeoning damage and is prone. If the characters succeed they take have damage and are not prone.

 Creat animated breaths(Costs 3 Actions). Blight sommons eight animated breaths(acid from only) in 20 ft of him.

Creat black puddling ( costs 6 actions). Blight summons 4 black puddlings in 20 ft of him.

Summon demon( costs 9 actions). Blight sommons 2 wastriliths in 20 ft of him.

Summon undead (cost 6 actions) light and summon 18 zombies, two zombie clots, four zombie plague spreaders, 18 crawling crawls, two ghouls, 8 specters, 8 shadows, or one zombie beholder.

Ran of acid and poison.This attack hits anyone in a 15 ft radius. Melee Weapon Attack: reach 15 ft, 15 target. Hit: 23 (2d10 + 3). On a successful save half damage.

Vortex to the abyss (Costs 12 Actions). A vortex of acidic water appears taking up 40 feet circle. If a player enters this space they take 57(5d10+7) acid damage and is teleported to the 2689 layer of the abyss,the player teleported returns on their next turn. After they come back they take 23(2d10+3) psychic damage and lands prone within 20 ft of the vortex. This vortex lasts for 3 turns. Turn one, 1 wastriliths and 2 black puddlings come to help Blight. Turn two, 1 young black dragon, 2 wastriliths, and 4 black puddlings come to help blight. Turn four, 1 adult black dragon, 2 young black dragons, 4 wastriliths, and 8 black puddlings come to help blight.