Regeneration: Adimarchus regains 15 hit points at the start of his turn. If Adimarchus is damaged by cold iron or a weapon of good alignment, this trait doesn't function at the start of his next turn.

Great Weapon Master: Before you make a melee attack with a heavy weapon that you are proficient with, you can choose to take a -5 penalty to the attack roll. If the attack hits, you add +10 to the attack's damage.

Improved Critical: Scores a critical hit on a natural roll of 19 or 20.

Staggering Critical: Knocks the target prone on a critical hit.

Madness: Adimarchus uses his Charisma modifier on Wisdom saves instead of his Wisdom modifier, and is immune to confusion and insanity effects.

Shroud Alignment: Spells and spell-like abilities with the good descriptor treat Adimarchus as if his alignment were good. Magic items are similarly fooled.

Weakness: Adimarchus has vulnerability to cold iron and weapons of good alignment

Mind Blank: Until the spell ends, one willing creature you touch is immune to psychic damage, any effect that would sense its emotions or read its thoughts, divination spells, and the charmed condition. The spell even foils wish spells and spells or effects of similar power used to affect the target’s mind or to gain information about the target.

Lie in State: If both of Adimarchus' forms die, Adimarchus (in his current form) disappears after 24 rounds. The body reappears dead but intact on Occipitus, his realm in the Abyss. Any equipment Adimarchus was wearing or carrying at the moment of his death is likewise transported to Occipitus - and is likewise reconstituted if it was destroyed. Dimensional anchor, dimensional lock, and similar spells delay the body's transportation in this manner, but once the spell's effect ends, the body transports to Occipitus immediately. After lying in state for 1d100 days on his home plane, both of Adimarchus's forms are restored to full life. If Adimarchus is slain on his home plane, he is dead forever.

Innate Spellcasting. Adimarchus' innate spell casting ability is Charisma (spell save DC 28). Adimarchus can innately cast the following spells, requiring no material components:

At will – detect good and evil, detect magic, teleport, truesight
3/day – darkness, dispel magic, plane shift (DC 28), unholy blight (DC 28)
1/day – horrid wilting (DC 28), maze, summon (1d3+1 glabrezus or 1 marilith), wall of force

Multiattack: Adimarchus makes two attacks: either with his Ashen Blade and/or his Tentacles

+5 Ashen Blade:  Melee Weapon Attack: +25 to hit, reach 5 ft., heavy, two-handed, 1 target. Hit: 23 (2d8 + 15) magical slashing damage. This greatsword appears to be made of ash and cinders. Once per round, a creature struck by the weapon must make a DC 28 Dexterity save or be immolated in a bloom of white-hot fire, taking 21d8 points of damage. Half of the damage is fire damage, and the other half is radiant damage that ignores resistance and immunity. A successful save halves the damage.

Enervating Bite: May make up to 4 separate attacks when used as an Attack. Melee Weapon Attack: +16 to hit, reach 10 ft., may split attacks among targets within reach. Hit: 12 (2d6 + 6) magical piercing damage. Any creature bitten by one or more of Adimarchus' tentacles has their max hit points reduced equal to the total amount of damage they received. The DC 28 Constitution save to remove this reduction can be made at the start of their next turn and each following turn.

Unholy Blight: You call up unholy power to smite your enemies. The power takes the form of a cold, cloying miasma of greasy darkness. Evil creatures are immune to this spell. The spell deals 5d10 necrotic damage to a good creature or 10d8 necrotic damage to a lawfully-good aligned character and causes it to be sickened for 2d4 rounds. A successful Wisdom save reduces damage to half and negates the sickened effect. The effects cannot be negated by remove disease, but spells that would remove a curse are effective. The spell deals only half damage to creatures who are neutral, and they are not sickened.

Horrid Wilting: You draw the moisture from every creature in a 30-foot cube centered on a point you choose within range (150 ft). Each creature in that area must make a DC 28 Constitution saving throw. Constructs and undead aren’t affected, and plants and water elementals make this saving throw with disadvantage. A creature takes 12d8 necrotic damage on a failed save, or half as much damage on a successful one. Non-magical plants in the area that aren’t creatures, such as trees and shrubs, wither and die instantly.

Maze: You banish a creature that you can see within range (60 ft) into a labyrinthine demiplane. The target remains there for the duration (10 minutes) or until it escapes the maze or you lose concentration on the spell. The target can use its action to attempt to escape. When it does so, it makes a DC 28 Intelligence check. If it succeeds, it escapes, and the spell ends (a minotaur or goristro demon automatically succeeds). When the spell ends, the target reappears in the space it left or, if that space is occupied, in the nearest unoccupied space.

Wall of Force: An invisible wall of force springs into existence at a point you choose within range (120 ft) and lasts up to 10 minutes or until you lose concentration. The wall appears in any orientation you choose, as a horizontal or vertical barrier or at an angle. It can be free floating or resting on a solid surface. You can form it into a hemispherical dome or a sphere with a radius of up to 10 feet, or you can shape a flat surface made up of ten 10-foot-by-10-foot panels. Each panel must be contiguous with another panel. In any form, the wall is 1/4 inch thick. It lasts for the duration. If the wall cuts through a creature’s space when it appears, the creature is pushed to one side of the wall (your choice which side). Nothing can physically pass through the wall. It is immune to all damage and can’t be dispelled by dispel magic. A disintegrate spell destroys the wall instantly, however. The wall also extends into the Ethereal Plane, blocking ethereal travel through the wall

BONUS ACTIONS:
Enervating Bite: May make up to 4 separate attacks when used as an Attack. Melee Weapon Attack: +16 to hit, reach 10 ft., may split attacks among targets within reach. Hit: 12 (2d6 + 6) magical piercing damage. Any creature bitten by one or more of Adimarchus' tentacles has their max hit points reduced equal to the total amount of damage they received. The DC 28 Constitution save to remove these negative levels can be made at the start of their next turn and each following turn.