Fireball Fly, Giant (5e Creature)
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Giant Fireball Fly
Small monstrosity, unaligned
Burning Proboscis. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 3 (1d4 + 1) piercing damage plus 7 (2d6) fire damage. If the target is a Small or larger creature, it is grappled, and the fly "Climbs Onto a Bigger Creature" (DMG page 271) .
Burn Up. The fireball fly releases chemicals inside its body that instantly reduces its hit points to 0.
Fireball. The fly can use this reaction when it is 0 hit points by damage other than cold, and it isn't killed outright. The fly explodes in a roaring ball of flame, dying in the process. Each creature within 20 feet of the fly must make a DC 12 Dexterity saving throw, taking 28 (8d6) fire damage on a failure, or half as much damage on a success. A creature has disadvantage on this save if it is grappled by the fly.
This monstrosity resembles a mosquito bloated to the size of a housecat, whose heat-shedding body glows like a torch.