Black Flame Cultist

Burning One's Blessing. When the Black Flame Cultist reduces a creature to 0 HP using Fire Damage they regain 1d12 Hit Points.

One With Shadows. When the Black Flame Cultist is in an area of dim light or darkness, they can use their action to become invisible until they move, take an action, or a reaction.

Burning Devotion. Spells cast by the Black Flame Cultist that deal fire damage bypass resistance and, have a doubled range, and ignores all but full cover.
Furthermore, any 1 rolled for fire damage is treated as a 2.

Black Flame Immolation (3/Day). As an action, the Black Flame Cultist can cause a target within 15 feet to become engulfed in pitch-black flames. The target must make a DC 16 Dexterity Save or take 15 (3d10) points of Fire Damage, and then take an additional 5 (1d10) points of damage for 1d6 turns afterward. On a success, only half damage is taken and the effect ends.
If a creature is brought to 0 HP while in this state, they automatically fail the first Death Saving Throw.
If a creature dies while under the effects of this spell, then their existence is forgotten by all except the closest to them.

Spellcasting. Black Flame Cultist is a 8th-level spellcaster. Black Flame Cultist's spellcasting ability is Charisma (spell save DC 16, +8 to hit with spell attacks). Black Flame Cultist has the following warlock spells prepared:

Cantrip (at will): create bonfire, fire bolt, green-flame blade
4th level (2 slots): burning hands, counterspell, fire shield, fireball, flaming sphere, hellish rebuke, mind spike, scorching ray, wall of fire

Multiattack. The Black Flame Cultist makes two attacks with their Flaming Dagger or Burning Blast

Flaming Dagger. Melee or Ranged Weapon Attack: +6 to hit, reach 5 ft. or range 20/60 ft., one target. Hit: 3 (1d4 + 2) piercing damage in melee, or 3 (1d4 + 2) piercing damage at range, plus 5 (1d10) fire damage.

Burning Blast. Ranged Spell Attack: +9 to hit, range 240 ft., one target. Hit: 6 (1d10 + 5) force OR fire damage (chosen when attack is rolled).

Arcane Desolation. When targeted by a ranged attack spell, the Black Flame Cultist can use their reaction to counter the spell with their own spell attack roll. If the Black Flame Cultist rolls the same number or higher than the original attack roll, the original spell fails, if they roll lower, the original spell goes through as normal.
If the countered spell is of 1st Level or Higher, the Black Flame Cultist can use a spell slot of that same level. If it hasn't been expended in 3 Rounds, they lose the spell slot.

Black Flame Cultist can take 2 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. Black Flame Cultist regains spent legendary actions at the start of its turn.

Chained Victim (1/Day). The Black Flame Cultist can call upon the phantom shade of a burned victim, which will follow the master in initiative. If the Cultist is killed, then the Shade will dissipate at the start of its next turn unless another Cultist uses this action.

Final Flames. When the Black Flame Cultist is brought to 0 HP, their body explodes into a torrent of pitch-black hellfire. Each creature within 15 feet must make a DC 18 Dexterity Saving Throw or take 24 (8d6) points of Fire Damage.