Attack Fly (5e Creature)
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Tiny beast, unaligned
Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one creature. Hit: 1 piercing damage.
An attack fly is an aggressive cousin of the black fly, with a distinctive shimmering red body. They can be found anywhere there is filth and decay. They will mindlessly pursue other living creatures but are only dangerous when they swarm.
It is not uncommon to find two conjoined flies. In such a state, they are known as a "moter" and act as a single creature with the above statistics, except with 2 hit points. If it is attacked with a weapon, it can use its reaction to split into two attack flies, each moving up to 5 feet without provoking an opportunity attack. The attack misses unless a fly remained in its original location.
For a swarm of attack flies, use the swarm of insects (Monster Manual p. 338): it has a walking speed of 5 feet, a fly speed of 20 feet, and no climbing speed. In addition, any creature hit by the swarm's bite must check for filth fever as above.