Balial

Regeneration. The creature regains 10 hit points at the start of its turn. If the creature takes cold or necrotic damage, this trait doesnt function at the start of the creatures next turn.

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

Rejuvenation. If the flameskull is destroyed, it regains all its hit points in 1 hour unless holy water is sprinkled on its remains or a dispel magic or remove curse spell is cast on them.

Innate Spellcasting.  Its spellcasting ability is Intelligence (spell save DC 16, +6 to hit with spell attacks). It requires no somatic or material components to cast its spells. The  has the following wizard spells prepared:

• (at will): mage hand
•  3/day: magic missile, shield
• 2/day: blur, flaming sphere
• 1/day each: fireball greater invisibility

Multiattack. The flameskull uses Fire Ray twice or makes two rapier attacks

Fire Ray. Ranged Spell Attack: +6 to hit, range 30 ft., one target. Hit: 10 (3d6) fire damage.

Rapier. Melee Spell Attack: +7 to hit, range 5 ftt., one target. Hit: (2d8+4) piercing damage.

Move. Moves up to its speed without provoking attacks of opportunity.

Rapier. Melee Spell Attack: +7 to hit, range 5 ftt., one target. Hit: (1d8+4) piercing damage.

Fir Ray. Ranged Spell Attack: +5 to hit, range 30 ft., one target. Hit: 10 (3d6) fire damage.