Darxktheolith Tag. The Zaskkartheo has the "(darxktheolith)" creature subtype, which acts the same way it would affect a creature than any other creature subtype.

Legendary Resistance (3/Day). If the Zaskkartheo fails a saving throw, it can choose to succeed instead.

Abyssal Void Resistance. The Zaskkartheo has advantage on saving throws against being stunned, knocked prone, frightened and incapacitated.

Greater Magic Resistance. The zaskkartheo automatically succeeds on saving throws against spells of 2nd level or lower (or similar magical effects), and has advantage on saving throws against all other spells or magical effects. 

Magic Weapons. The zaskkartheo's weapon attacks are considered magical.

Void Sight. Magical darkness does not impede the zaskkartheo's darkvision.

Charge. If the zaskkartheo moved at least 20 feet toward a target right before making a melee attack, the attack deals an additional 13 (2d12) damage and the target must succeed on a DC 19 Strength saving throw or be knocked prone.

Lesser Regeneration. The zaskkartheo regains 20 hit points at the start of each of its turns if it is at half-hit points or lower.

Aura of Sinful Malevolence. Whenever a good-aligned creature starts its turn while within 60 feet of and can hear the zaskkartheo, the must succeed on a DC 19 Charisma saving throw or have their alignment move one step toward being chaotic evil. Once a good-aligned creature fails this save five times, their alignment automatically becomes chaotic evil and they become susceptible to the aura's charming affect.  Whenever a evil-aligned fiend, monstrosity, or celestial starts its turn within 90 feet of and can hear the zaskkartheo must succeed on a DC 21 Charisma saving throw or be charmed by the zaskkartheo for 1 minute. The charmed creature can attempt to repeat the save at the start of each of its turns, taking 13 (2d12) psychic damage on a failure, or no damage and removing the effect on a success.   The zaskkartheo's aura has a different effect depending on what sin or conceptual evil it represents. The DM can choose exactly what evil the zaskkartheo has and what effect its aura does, but here are some examples: 

• Greed: Creatures hostile to the zaskkartheo that start their turn(s) within 90 feet of the zaskkartheo have disadvantage on Charisma saving throws and ability checks and on attack rolls as long as they can see or have the knowledge of the location of an object worth at least 1 sp.
• Wrath/War: Creatures hostile to the zaskkartheo that start their turn(s) within 90 feet of the zaskkartheo have advantage on attack rolls and must make a melee or ranged attack at the start of each of their turns, or they will have disadvantage on Wisdom, Charisma, and Intelligence checks and saving throws for 1 minute.
• Envy: Creatures hostile to the zaskkartheo that start their turn(s) within 90 feet of the zaskkartheo that has a numerical bonus to any one of their stats have to succeed on a DC 17 Intelligence saving throw at the start of each of the zaskkartheo's turns or have that bonus stolen by the zaskkartheo. The zaskkartheo then has that bonus for half the normal time that the creature it stole it from did (this only applies to bonuses that last for minutes or longer, bonuses that last for 1 round the zaskkartheo has for the same duration).
• Gluttony: Creatures hostile to the zaskkartheo that start their turn(s) within 90 feet of the zaskkartheo have advantage on melee attack rolls against their allies and must deal damage to some target on each of their turns or take 7 (2d6) poison damage which they cannot resist.
• Famine: Creatures hostile to the zaskkartheo that start their turn(s) within 90 feet of the zaskkartheo have disadvantage on saving throws against exhaustion and take an amount of necrotic damage equal to half the amount of hit points healed whenever a spell is cast on them or similar effect is applied to them that heals hit points. 
• Pestilence: Creatures hostile to the zaskkartheo that start their turn(s) within 90 feet of the zaskkartheo have disadvantage on all saving throws and must succeed on a DC 16 Constitution saving throw at the start of each of their turns or be poisoned for 1 minute.
• Death: Creatures hostile to the zaskkartheo that start their turn(s) within 90 feet of the zaskkartheo automatically fail the first death save they make in combat, cannot regain hit points nonmagically, and whenever they regain hit points magically (via a spell, magic item, or similar magical source) the amount of hit points healed is halved.

Multiattack. The zaskkartheo makes three attacks: one with its bite or tail and two with its claws or halbred. It can replace any one of these attacks with a Spear attack or a casting of one of its at-will spells.

Bite. Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 17 (2d8 + 9) piercing damage + 14 (4d6) necrotic or poison damage (zaskkartheo's choice) and the target has its hit point maximum decreased by half the amount of necrotic or poison damage dealt. This reduction lasts until cured by a greater restoration spell or similar magic.

Horns. Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 18 (2d8 + 9) piercing damage + 11 (2d10) force damage, and the target, if it is Large or smaller, must succeed on a DC 20 Strength saving throw or be pushed 5 feet away from the zaskkartheo.

Claw. Melee Weapon Attack: +16 to hit, reach 15 ft., one target. Hit: 16 (2d6 + 9) slashing damage + 18 (4d8) necrotic damage.

Tail. Melee Weapon Attack: +16 to hit, reach 15 ft., one target. Hit: 15 (1d12 + 9) bludgeoning damage + 9 (2d8) necrotic damage.

Sinful Spear. Melee Weapon Attack: +17 to hit, reach 15 ft. or range 35/80 feet, one target. Hit: 17 (2d6 + 10) piercing damage, or 19 (2d8 + 10) piercing damage if used with two limbs to make a melee attack, plus 29 (8d6 + 1) necrotic damage. The spear immediately returns to the zaskkartheo's hands after it makes a ranged attack with it.

Sinful Halberd. Melee Weapon Attack: +17 to hit, reach 20 ft., one target. Hit: 21 (2d10 + 10) slashing damage + 29 (8d6 + 1) necrotic damage.

Malicious Sacrifice (Recharge 6). The zaskkartheo targets one of its allies to bolster itself and get rid of any sort of weight the ally might be giving it in battle. That ally must succeed on a DC 23 Wisdom or Charisma saving throw (target's choice) or have it that at the start of each turn in combat, the target has its hit points decreased by 4d10, with its hit point maximum being decreased by half that amount at the same time and is incapacitated for 1 minute. Every time the target loses hit points in this way, the zaskkartheo regains hit points equal to half the amount the target's hit point maximum was decreased and also regains one non-at-will, non-1/day spell slot. The draining effect lasts until the end of the next round, until the target has its standard hit points reduced to 0, or when the zaskkartheo ends it (no action required).

Sinful Swath (Recharge 3-6). The zaskkartheo swings its sinful halbred in a massive arc while powering it up with terrifying necrotic energy, forcing all creatures within 25 feet of it that are right in front of it to make a DC 22 Dexterity saving throw, taking 21 (2d10 + 10) slashing damage + 36 (10d6 + 1) necrotic damage on a failed save, or half as much on a success.

Summon Fiends (1/day). The zaskkartheo attempts a magical summoning. It has a 80% chance to summon 1d6 sipulxxheos, 1d8 duzekuzdheos, 1d12 mezzoloths, 1d8 glabrezus, 2d4 vrocks, or 2d6 bulezaus. A summoned fiend appears in an unoccupied space within 60 feet of its summoner, acts as an ally of its summoner, and can't summon other fiends. It remains for 1 minute, until it or its summoner dies, or until its summoner dismisses it as an action.

Spellcasting. The Zaskkartheo's spellcasting ability is Charisma (spell save DC 23, +15 to hit with spell attacks). The zaskkartheo can innately cast the following spells, requiring no material components:

At will: detect magic, fireball (as 6th level spell), wall of fire, darkness, animate dead, invisibility, protection against evil and good (celestials, fey, aberrations, and monstrosities only), hellish rebuke (as a 6th level spell), detect evil and good, antilife shell

3/day each: insect plague, power word pain, plane shift, dominate person, circle of death (can deal necrotic, fire, force, aetherion, poison, or lightning damage)

6/day: power word kill, power word stun, teleport (targets do not have to be willing to be affected)

3/day: divine word

Teleport. The zaskkartheo and up to three other creatures and all items it is wearing or carrying teleport to an unoccupied space the zaskkartheo can see within 600 feet of it.

Change Shape. The zaskkartheo magically polymorphs into a humanoid, beast, or monstrosity 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 (zaskkartheo's choice). In a new form, the zaskkartheo retains its alignment, hit points, Hit Dice, ability to speak, proficiencies, Legendary Resistance 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.

Abyssal Chains (Recharge 4-6). In response to being restrained, grappled, poisoned, or having its movement speed decreased, the zaskkartheo has chains of blazing eldritch magic surround and entrap the creature that the zaskkartheo believes afflicted it with the condition. That creature must succeed on a DC 22 Dexterity saving throw or be restrained for 1 minute. While restrained in this way, the target has disadvantage on all saving throws and takes 10 (3d6) fire damage + 4 (1d8) aetherion damage at the start of each of its turns. The creature can attempt the saving throw again on its turns (except that it is a Dexterity save instead of Strength), freeing itself on a success. 

The zaskkartheo can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature's turn. The zaskkartheo regains spent legendary actions at the start of its turn.

Strike. The zaskkartheo makes a Claw, Tail, or Spear attack.

Spell of Sin. The zaskkartheo casts one of its at-will spells.

Teleport. The zaskkartheo uses its Teleport.

Wing Attack (Costs 2 Actions). The zaskkartheo flaps its monstrous wings. Each creature within 10 feet of it must make a DC 23 Dexterity saving throw, taking 16 (2d6 + 9) bludgeoning damage + 7 (2d6) necrotic damage and be knocked prone on a failure, or half as much damage on a success. The zaskkartheo can then fly up to half its flying speed.

Magic of The Pit (Costs 3 Actions, 1/Short or Long Rest). The zaskkartheo draws on the terrifying powers of the Howling Void, allowing it to cast one necromancy, illusion, evocation, abjuration, or enchantment spell of 7th level or lower of its choice.