Regeneration: Adimarchus regains 10 hit points at the start of his turn. If Adimarchus is damaged by radiant damage, this trait doesn't function at the start of his next turn.

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.

Innate Spellcasting. Adimarchus' innate spell casting ability is Charisma (spell save DC 20). 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 20), unholy blight (DC 20)
1/day – horrid wilting (DC 20), maze, summon (1d4 glabrezus or 1 marilith), wall of force

+5 Ashen Blade:  Melee Weapon Attack: +15 to hit, reach 5 ft., heavy, two-handed, 1 target. Hit: 17 (2d8 + 10) 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 20 Dexterity save or be immolated in a bloom of white-hot fire, taking 6d10 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 3 separate attacks when used as an Attack. Melee Weapon Attack: +9 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 20 Constitution save to remove this reduction can be made at the start of their next turn and each following turn.

Horrid Wilting: You draw the moisture from every creature in a 30-foot cube centered on a point you choose within range (100 ft). Each creature in that area must make a DC 20 Constitution saving throw. Constructs and undead aren’t affected, and plants and water elementals make this saving throw with disadvantage. A creature takes 6d8 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.