Cursed Deep gnome King (spell caster)

Undying Form.
If reduced to 0 HP and isn't killed the same way he originally died (see Echo of Death), he instead drops to 1 HP and becomes Incorporeal until the start of his next turn. This ability recharges after a long rest. 

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

Arcane Warding.
As long as he has at least 1 hit point, he adds his Charisma modifier (+6) to his AC and saving throws against spells.

Undying (Curse)

Can only be killed from the first method of death. When killed he regains 1/4th of his hp except the first time and regains 2 spell slots of your choice (Don't TPK your party!!)

Ex if killed from sword slash has to be killed from sword slash

Spellcasting.

An 18th-level spellcaster (Charisma-based, DC 21, +13 to hit). His spell list blends sorcerer, warlock, and custom necromancy.

Cantrips (at will): Mage Hand, Toll the Dead, Minor Illusion, Eldritch Blast (3 beams, +6 necrotic each)
1st (4 slots): Shield, Hex, Mage Armor
2nd (3 slots): Mirror Image, Misty Step
3rd (3 slots): Counterspell, Dispel Magic, Vampiric Touch
4th (3 slots): Greater Invisibility, Shadow of Moil
5th (3 slots): Wall of Force, Synaptic Static
6th (1 slot): Eyebite
7th (1 slot): Finger of Death
8th (1 slot): Dominate Monster

Actions

Multiattack.

he can cast one spell and make one Voidlash attack, or cast two cantrips.

Voidlash.

Melee Spell Attack: +13 to hit, reach 10 ft., one target.
Hit: 21 (4d8 + 3) necrotic damage. The target must succeed on a DC 21 Wisdom saving throw or be blinded until the start of Deltrax’s next turn as shadows engulf their vision.

Command the Shadows (Recharge 5–6).

He targets up to 3 creatures within 60 ft. Each target must make a DC 21 Constitution save or take 45 (10d8) necrotic damage and be grappled by living shadow (escape DC 19). Grappled creatures are restrained and take 10 (2d8) necrotic damage at the start of each of their turns.

Siphon Soul (Recharge 6).

He targets one creature he can see within 60 feet. That creature must make a DC 21 Charisma saving throw. On a fail:

• They take 66 (12d10) psychic damage.

• Deltrax heals for half the damage dealt.

• The creature cannot regain hit points until the end of its next turn.

Phase Shift (1/Day).

He becomes incorporeal for 1 minute. During this time:

• He gains resistance to all damage except radiant, force, and psychic.

• He can move through solid objects as if they were difficult terrain.

• His Voidlash deals an extra 2d8 psychic damage.

Bonus Actions

Misty Veil (Recharge 4–6)

He teleports up to 30 feet to a space he can see and becomes heavily obscured in magical shadow until the start of his next turn. Until then, attacks against him are made with disadvantage, and he has advantage on Dexterity saving throws.

Hex of the Hollow King (1/Short Rest)

curses one creature he can see within 60 ft. Until the end of his next turn:

• That creature takes an extra 2d6 necrotic damage whenever it takes damage.

• If it drops to 0 HP during that time, it rises as a shadow under his control for 1 minute (use Shadow stat block).

Surge of the Undying (2/Day)

He regains 25 HP and ends one condition currently affecting him.

Reactions

Vengeful Glare (Recharge 5–6)

When a creature within 60 feet hits him with an attack, he can force them to make a DC 21 Wisdom saving throw. On a failure, they are frightened until the end of their next turn and take 13 (3d8) psychic damage.

Shadow Rebuke (3/Day)

When he is hit by a melee attack, he unleashes a retaliatory burst of shadow. The attacker must succeed on a DC 21 Dexterity saving throw or take 22 (4d10) necrotic damage and have their movement halved until the end of their next turn.

Spell Mirror (1/Day)

When targeted by a spell of 5th level or lower that only targets him, he may choose to reflect it back at the caster as if they had cast it on themselves. He must use his reaction immediately and can choose to expend a spell slot of the same level to amplify it (e.g., upcast).

Legendary Actions (3/round)

Shadowstep.
Cursed Deep Gnome kingteleports up to 30 feet to an unoccupied space he can see and becomes invisible until the start of his next turn.

Psychic Rupture (Costs 2).
Each creature of his choice within 30 feet must make a DC 21 Int save or take 4d10 psychic damage and be stunned until the end of their next turn on a fail.

Cast a Cantrip.
Cursed Deep Gnome king casts a cantrip.

Mythic Trait: Echo of Death (Activates at 0 HP, once per encounter)

When reduced to 0 HP for the first time, Deltrax explodes into black smoke and vanishes. On initiative count 20 (next round), he reforms in a shadowy burst with half his hit points (138 HP), all spells restored, and a new action:

True Death Shroud (1/day): For 1 minute, any creature that reduces another creature to 0 HP within 60 ft. of Deltrax must succeed on a DC 21 Wisdom save or take 6d10 necrotic damage and become frightened for 1 minute.