Flyby: Xomalai's flight doesn't provoke opportunity attacks

Fiendish Weapons. Xomalai's weapon attacks are magical. When Xomalai hits with any weapon, the weapon deals an extra 6d8 necrotic damage (included in the attack).

Xomalai can use a bonus action to conjure the Greatsword or Draining Longbow from its demiplane. Both can be active. Draining Longbow creates its own ammunition when drawn.

Evil Awareness. Xomalai knows if it hears a lie.

Observant War Caster. Xomalai has the Observant and War Caster feats.

Innate Spellcasting. Xomalai's spellcasting ability is Charisma (spell save DC 23, spell attack +15). It can innately cast the following spells, requiring no material components:

At will: Darkness, Thunderwave at 3rd level.
Lightning Lure, Fire Bolt, Frostbite Toll the Dead. All cantrips cast as a 17th level caster.

3/day each: Lightning Bolt (at 5th level), Fireball (at 5th level), Shatter (at 5th level), Cone of Cold

1/day each: Call Lightning (at 6th level), Circle of Death, Finger of Death, Disintegrate

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

Reach. Xomalai's touch and weapon reach is 10ft.

Shadow Regeneration. As an action, Xomalai eminates a dark aura of regeneration cascading down like a robe of darkness around its body. Considered a 9th-level spell, the regeneration does 38 (5d10+10) healing at the start of each of his turns.

If Xomalai is hit by a melee attack or ranged attack within 60ft, it interrupts the regeneration on its turn but the darkness retaliates instead. Up to 5 targets that hit that are still within 60ft at the start of Xomalai's turn divide the 38 (5d10+10) healing between them as necrotic damage.

This ability lasts until dispelled, and recharges at dusk unless it is currently active.

Legendary Resistance (4/Day). If Xomalai fails a saving throw, it can choose to succeed instead.

Multiattack. Xomalai makes three weapon attacks.

Greatsword. Melee Weapon Attack: +16 to hit, reach 5 ft., one target. Hit: 34 (6d6 + 10) slashing damage plus 27 (6d8) necrotic damage.

Draining Longbow. Ranged Weapon Attack: +14 to hit, range 150/600 ft., one target. Hit: 22 (3d8 + 7) piercing damage plus 27 (6d8) necrotic damage. If the target is a humanoid creature, it must succeed on a DC 15 Constitution saving throw or suffer one level of exhaustion.

Flying Sword.  As a bonus action, Xomalai releases its greatsword to hover magically in an unoccupied space within 5 feet of it. If Xomalai  can see the sword, Xomalai can mentally command it as a bonus action to fly up to 50 feet and either make one attack against a target or return to Xomalai's hands. If the hovering sword is targeted by any effect, Xomalai is considered to be holding it. The hovering sword falls if the Xomalai dies.

Healing Touch (4/Day). Xomalai touches another creature. The target magically regains 40 (8d8 + 4) hit points and is freed from any curse, disease, poison, blindness, or deafness.

Consume: Xomalai can consume one magic item (Strength check, DC varies) or soul (Charisma Check, DC 20) it is currently in possession of.

A magic item consumed this way will grant healing. 4d4+4 for a common item (no save), 8d6+8 for uncommon (no save), 16d8+16 for rare (DC 20), 32d10+32 for very rare (DC 25), and 64d12+64 (DC 30) for a legendary. An artifact cannot be consumed in this way.

A soul is treated as a rare item.

Soul Cage. Xomalai can cast Soul Cage without expending spell slots or any components. This magically absorbs the body of the target as well.

Its body acts as the cage for a soul, with the ability to hold up to 5 souls at a time. Using an action, Xomalai can channel the energy of creature's living form, acquiring class abilities (including spellcasting abilities, spell slots, ki abilities, sorcery points, any metamagic options, etc) the soul still had at the time of capture.

The soul can remain contained for up to 5 days before either being consumed or breaking out of its cage and being free to its normal afterlife duties. 

War Caster. Xomalai can use its opportunity attack to cast a spell with a single target.

Xomalai can take 4 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. Xomalai regains spent legendary actions at the start of its turn.

React. (Costs 1 Action) Xomalai can use 1 Legendary Action to regain an expended Reaction. 

Teleport. (Costs 1 Action) Xomalai magically teleports, along with any equipment it is wearing or carrying, up to 120 feet to an unoccupied space it can see.

Elemental Burst (Costs 2 Actions). Xomalai emits magical, divine energy. Each creature of its choice in a 10-foot radius must make a DC 23 Dexterity saving throw, taking 10 (3d6) fire damage, 10 (3d6) cold damage, 10 (3d6) thunder damage, and 10 (3d6) lightning damage on a failed save, or half as much damage on a successful one.

Gaze of The Darkness (Costs 2 Actions). Xomalai targets one creature it can see within 30 feet of it. If the target can see it, the target must succeed on a DC 15 Constitution saving throw or be blinded until magic, such as the lesser restoration spell, removes the blindness.

Cast Spell, 3rd Level or Lower (Costs 1 Action)

Cast Spell, 4th or 5th Level (Costs 2 Actions)

Cast Spell, 6th or 7th Level (Costs 3 Actions)

Cast Spell, 8th or 9th (Costs 4 Actions)