Illumination. The bomb 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.

Fire Form.  A creature that touches the bomb or hits it with a melee attack while within 5 feet of it takes 5 (1d10) fire damage. The first time it enters a creature's space on a turn, that creature takes 5 (1d10) fire damage and catches fire; until someone takes an action to douse the fire, the creature takes 5 (1d10) fire damage at the start of each of its turns.

Innate spellcasting. The bomb is a 6th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 14, +6 to hit with spell attacks). It requires no somatic or material components to cast its spells. The flameskull has the following wizard spells prepared:

At will: fire bolt
3/day: scorching ray
1/day: fireball

Self Destruct. When the bomb has inflated three times, it self destructs. Any creature within 30 ft. of the bomb must make a DC 14 dexterity saving throw, taking 28 (8d6) fire damage on a failed save, or half as much on a successful one.

Bash. Melee weapon attack: +4 to hit, one target. Hit: 4 (1d6 + 1) bludgeoning damage plus 5 (1d10) fire damage.

Inflate. In response to being hit by a melee weapon attack or a ranged weapon attack, the bomb inflates one size larger. When it inflates three times is uses its self destruct.