Armor Class 13
Hit Points 40 (9d4 + 18)
Speed fly 40 ft.
Skills Arcana +5, Perception +2
Damage Resistances lightning, necrotic, piercing
Damage Immunities cold, fire, poison
Condition Immunities charmed, frightened, paralyzed, poisoned, prone
Senses darkvision 60 ft., passive Perception 12
Languages Common
Challenge 4
Illumination. The flameskull sheds either dim light in a 15-foot radius, or bright light in a 15-foot radius and dim light for an additional 15 feet. It can switch between the options as an action.
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.
Multiattack. The flameskull uses Fire Ray twice.
Fire Ray. Ranged Spell Attack: +5 to hit, range 30 ft., one target. Hit: 10 (3d6) fire damage.
Monster Manual (2014), p. 134