Blackwell

Legendary Resistance (10/Day): If Blackwell fails a saving throw, it can choose to succeed instead.

Formless. Being a member of the 4 Elders, Blackwell has no true form. His preference is to interact with people as a disembodied voice or better yet, someone he knows the person can be easily manipulated by. The other form he prefers is a humanoid with demonic paint on his face dressed in black armor adorned in spikes.

Negative Energy Cone. Blackwell emits an invisible, magical 60-ft aura of negative energy. At the start of each of its turns, the tyrant decides whether or not he wants the aura to be active. Any creature in that area can't regain hit points. Any humanoid that dies there becomes a Creature of the Night under the tyrant's command. The dead humanoid retains its place in the initiative order and animates at the start of its next turn, provided that its body hasn't been completely destroyed.

Avoidance. If Blackwell is subjected to an effect that allows it to make a saving throw to take only half damage, it instead takes no damage if it succeeds on the saving throw, and only half damage if it fails.

Turn Immunity. Blackwell is immune to effects that turn undead.

Illumination. Blackwell sheds either dim light in a 15-foot radius, or bright light in a 15-foot radius and dim light for an additional 15 feet. It can switch between the options as an action.

Magic Resistance. Blackwell has advantage on saving throws against spells and other magical effects.

Shapechanger. Blackwell can use its action to polymorph into a Small or Medium humanoid it has seen, or back into its true form. Its statistics, other than its size, are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.

Ambusher. Blackwell has advantage on attack rolls against any creature it has surprised.

Surprise Attack. If the Blackwell surprises a creature and hits it with an attack during the first round of combat, the target takes an extra 10 (3d6) damage from the attack

Spellcasting. Blackwell is a 100th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 59, +51 to hit with spell attacks). Blackwell has following the following Bard, Cleric, Druid, Paladin, Ranger, Sorcerer, Warlock, and Wizard spells prepared:
Cantrips (at will): All
1st level (100 slots): All
2nd level (50 slots): All
3rd level (25 slots): All
4th level (12 slots): All
5th level (6 slots): All
6th level (3 slots): All
7th level (1 slot): All
8th level (1 slot): All
9th level (1 slot): All
10th level (1 slot): Animus Blast

Be advised that because he is and primordial being, Blackwell can use any weapon or ability he wants at any time. However, because he embodies vengeance, terror, and the darkest impulses of mortal souls he tends to prefer weapons and abilities that have a more sinister reputation. The following list is just examples of things Blackwell could employ against his enemies.

Abyssal Bleed. A variety of Blackwell's attacks will apply this effect. Dark energy will form wherever the receiver got hit by an attack that causes this effect. At the end of each of a bleeding targets turn, they will take 2 damage. This will stack by 1 for each additional time they are struck by another attack which causes this effect. This will happen till the bleeding is cured or Blackwell has been defeated. (Abyssal Bleed can be removed with any 4th level spell or higher that cures diseases).

Abyssal Combustion. With every fourth of his HP depleted he will take in the surrounding dark aura and expel it in a huge blast. Anyone within 30 ft. of Blackwell makes a DEX Saving throw against a spell save DC of 59. On a failed save they take 33 (6d10) necrotic damage or half as much on a success. (On death one last Abyss Combustion will occur.) If multiple thresholds are met simultaneously the equivalent number of combustions occur at once. Applies Abyssal Bleed.

Abyss Walker. After the first Abyss Combustion Blackwell will start to leave behind a trail of black necrotic energy in his wake. The trail does not impede movement but if a creature moves through it they will take (1d4) necrotic damage. After every sequential Abyss Combustion, the damage will increase by another 1d4. The black energy as a condensed gas, not flammable but can be move with high winds or if an AOE centric Radiant spell hits an area with it the energy will dissipate. If a creature ends their turn in the gas it Applies Abyssal Bleed

Erupting Grasp. One target within 5 feet must make a DC 59 Dexterity saving throw. On a failed save, Blackwell grabs the target at the throat and releases a fiery explosion from his palm. The target takes 17 (2d8+8) bludgeoning damage and 27 (6d8) fire damage.

Firestorm. Blackwell kindles the remains of some of the souls he has collected and releases a burst of fire. All creatures within 20 feet must make a DC 59 Dexterity saving throw, taking 36 (8d8) fire damage on a failed save, or half as much damage on a successful one.

KISS of Dr. Love. A grappled target must succeed a DC 59 constitution saving throw or take 27 (5d8 + 5) necrotic damage, the target takes half damage on a successful saving throw. If a target is reduced to 0 hit points by this attack, instead of dying, it becomes a Creature of the Night after 1d4 days. A Remove Curse or Greater Restoration spell can prevent this transformation, stabilizing the target at 0 hit points. Once the transformation is complete, nothing short of a wish spell can reverse it.

War Machine. Roll a D20, on even rolls Blackwell summons an Imperial War Machine (see homebrew monsters list) on odd rolls he summons an Infernal War Machine (again, see homebrew monsters list). If crew members are needed, a manifestation of the Creatures of the Night will take the place of the crew. These souls will appear as black gaseous humanoids with red eyes and no other facial features. They do not speak or do anything other than what is required to run the War Machine and will disappear if the War Machine is destroyed. Blackwell will always Teleport away from the battle when summoning his War Machine as he feels this is beneath him and is confident his War Machines will destroy everything in their path. 

Magical War Hammer. Blackwell summons a magical war hammer as part of his attack which disappears right after. 
Melee Weapon Attack: +16 to hit, reach 10 ft., one creature. Hit: 36 (6d8 + 9) magical bludgeoning damage. Upon a hit, the creature must make a DC 59 CON Saving Throw and be incapacitated on a failed save until the end of their next turn.

Leaping War Hammer. Blackwell leaps 30ft and attacks with his Magical War Hammer. If the attack hits, the target must make two DC 59 CON Saving Throws instead of 1. If the target passes both, no additional effect is received. If the target fails 1 they are stunned until the end of their next turn. If the target fails both they are stunned and the attack becomes a critical hit.

Great Fireball.  Blackwell manifests a large chaos-infused fire orb. Each creature within a 40ft radius of the center of a sphere must succeed on a DC 59 Dexterity saving throw, or take 8d8 fire damage on a failed save, or half as much on a successful one. 

Abyssal Soul Spear. Blackwell manifests a spear of pure darkness into his palm and hurls it at a single target. The target must succeed on a DC 59 Dexterity saving throw or take 6d8 necrotic damage and be stunned on a failed save, or half as much damage and avoid the stunned condition on a successful save.

True God of Thunder. A bolt of lightning 5ft wide and 100ft long shoots from Blackwell, each creature in the line must make a DC 59 Dexterity saving throw. A creature takes 72 (16d8) lightning damage on a failed save, or half as much damage on a successful one. A creature that fails the Dexterity Save must make a DC 59 Constitution Save or be Stunned for one turn.

Trembling Strike. Blackwell strikes the ground with a bolt of lightning, triggering an earth tremor with bolts of lighting streaking across the ground. All other creatures on the ground within 60 feet of Blackwell must succeed on a DC 59 Strength saving throw or be knocked prone.

Unholy Drain. Blackwell targets up to three creatures that it can see within 10 feet of it. Each target must succeed on a DC 59 Constitution saving throw or take 21 (6d6) necrotic damage, and Blackwell regains hit points equal to the total damage dealt to all targets.

Multi-Attack. Blackwell can use its Frightful Presence. It then makes three attacks.

Frightful Presence. Each creature of Blackwell's choice that is within 120 feet of Blackwell and aware of it must succeed on a DC 59 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 Blackwell's Frightful Presence for the next 24 hours.

Breath Weapons (Recharge 5–6). Blackwell uses one of the following breath weapons.

Fire Breath. Blackwell exhales fire in a 90-foot cone. Each creature in that area must make a DC 59 Dexterity saving throw, taking 71 (13d10) fire damage on a failed save, or half as much damage on a successful one.

Weakening Breath. Blackwell exhales gas in a 90-foot cone. Each creature in that area must succeed on a DC 59 Strength saving throw or have disadvantage on Strength-based attack rolls, Strength checks, and Strength saving throws 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.

Change Shape. Blackwell magically polymorphs into a humanoid or beast that has a challenge rating no higher than its own, or back into its true form. It reverts to its true form if it dies. Any equipment it is wearing or carrying is absorbed or borne by the new form Blackwell's choice). In a new form, Blackwell retains its alignment, hit points, Hit Dice, ability to speak, proficiencies, Legendary Resistance, lair actions, and Intelligence, Wisdom, and Charisma scores, as well as this action. Its statistics and capabilities are otherwise replaced by those of the new form, except any class features or legendary actions of that form.

Read Thoughts. Blackwell magically reads the surface thoughts of one creature within 60 feet of it. The effect can penetrate barriers, but 3 feet of wood or dirt, 2 feet of stone, 2 inches of metal, or a thin sheet of lead blocks it. While the target is in range, Blackwell can continue reading its thoughts, as long as Blackwell concentration isn't broken (as if concentrating on a spell). While reading the target's mind, Blackwell has advantage on Wisdom (Insight) and Charisma (Deception, Intimidation, and Persuasion) checks against the target.

Action Melee Attack. Multiattack. Blackwell makes two melee attacks.

Slam. Melee Weapon Attack: +45 to hit, reach 5 ft., one target. Hit: 7 (1d6 + 45) bludgeoning damage.

Action Ranged Attack. Fire Ray. Ranged Spell Attack: +45 to hit, range 30 ft., one target. Hit: 10 (3d6) fire damage.

Enter the description for how bonus actions work for your monster here.

Reaction Name. N/A?

Living in Sin. Should Blackwell find a mortal amusing, attractive or otherwise entertaining, Blackwell can transport them at will to his Lair on the grounds of KISS Castle. He can also do this to those he wishes to personally witness suffer for all eternity.