Fireball Fly Swarm (5e Creature)
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Fireball Fly Swarm
Medium swarm of tiny beasts, unaligned
Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a Tiny insect. The swarm can't regain hit points or gain temporary hit points.
Fireball Burst. If the swarm suffers a critical hit from an attack that did not deal cold damage, one of the flies comprising the swarm erupts in a 5-foot radius ball of fire. Each creature in that area must make a DC 13 Dexterity saving throw, taking 7 (3d4) fire damage on a failed save, or half as much damage on a successful one.
Multiattack. The swarm makes one bite attack. It then uses its Suicide Dive if it doesn't have all its hit points.
Bites. Melee Weapon Attack: +3 to hit, reach 0 ft., one target in the swarm's space. Hit: 10 (4d4) piercing damage, or 5 (2d4) piercing damage if the swarm has half of its hit points or fewer.
Suicide Dive. If the swarm doesn't have all its hit points, it can sacrifice one fireball fly. The swarm loses 2 hit points. The sacrificial fly strikes a point within 5 feet of the swarm and erupts per Fireball Burst.
These insects are infused with elemental fire and can be found swarming in rocky wilderness areas, particularly near active volcanoes or magma streams. Like bees, individual fireball flies will sacrifice themselves to protect the swarm. They do so by immolating themselves in a burst of fire.