Illumination and Perception. The flameskull sheds dim blue light in a 15-foot radius. It has Darkvision to 60 feet. It has skill in Perception (+3) and Arcana (+4).

Resistance and Immunities. The flameskull is resistant to necrotic and fire damage. It is immune to poison damage. It may not be grappled or made prone. It cannot be charmed or frightened.

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.

Spellcasting. The flameskull is a 3rd-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 13, +3 to hit with spell attacks). It requires no somatic or material components to cast its spells. The flameskull has the following wizard spells prepared:

Cantrip (at will): Chill Touch

1st level (4 slots): Witch Bolt, Shield

2nd level (2 slots): Shatter

Howling Alert. The howling skull briefly sheds bright light in a 30-foot radius and dim light in an additional 30-foot radius until the beginning of its next turn. It also howls loudly in a fashion audible to creatures within 500 feet. It will frequently use this as its first action in an encounter to signal undead allies. 

Multiattack. The howling skull uses Vicious Howl twice. It may only do this against targets within the same 90-degree facing.

Vicious Howl. Ranged Spell Attack: +3 to hit, range 30 ft., one target. Hit: 6 (2d6) thunder damage.

Shriek of the Dead. Each creature in a 15-foot cube centered around the Howling Skull must make a Constitution DC 13 saving throw. On a failed save, a creature takes 10 (2d10) necrotic damage. On a successful save, the creature takes 5 (1d10) damage.