Vincaghathrax

Armor of Shadows. Vincaghathrax can cast mage armor and disguise self at will, without expending a spell slot.

Pact Magic. Vincaghathrax is a 9th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 17, +9 to hit with spell attacks). Vincaghathrax knows the following warlock spells, and has 2 5th level spell slots:
Cantrips (at will): eldritch blast, minor illusion, scorch, shadowthorn 
1st level: burning hands, command

2nd level: blindness/deafness, scorching ray

3rd level: fireball, stinking cloud

4th level: fire shield, wall of fire

5th level: flame strike, hallow

Dark One's Blessing. Whenever Vincaghathrax reduces a creature to 0 hit points, he gains 14 temporary hit points.

Blessing of the Dragon Queen. Vincaghathrax does and extra 2d8 necrotic damage when attacking with his Longblade(included in the attack).

Legendary Resistance (1/day). If Vincaghathrax fails a save, it can choose to succeed it instead.

Assault. Vincaghathrax makes 2 attacks with his longblade, or makes one longblade attack and casts a cantrip. He can replace any melee attack with his breath weapon if available.

Longblade. Melee Weapon Attack: +7 to hit, reach 5 ft., 1 target. Hit: 9 (1d10 + 4) slashing damage and 10 (2d8) necrotic damage.

Necrotic Breath (Recharge 6). Vincaghathrax exhales a necrotic field in a line that is 60 ft. long and 5 ft. wide, or a 30 ft. cone. Every creature in that area must make a DC 17 Constitution saving throw, taking 3d10 necrotic damage on a failed save, or half as much on a success.