Fyrefly

Adults are big, unhurried black insects with red eyes and red wings. Larvae look like tiny insect-shaped motes of intense flame.

Adults are harmless. Only larvae attack living things, and only right after hatching, when they go into a frenzy and hurl themselves at anything that moves. In its frenzy a larva fights as a 5 HD monster, THAC0 15, and is so fast it counts as AC 5, or AC 9 if somehow at rest. Fire cannot harm larvae.

Any hit by an adult or larva may land on something burnable. The DM estimates how much of the target is flammable, perhaps 10% of a fighter's gear but 90% of a wizard's, and rolls percentile dice. A flammable item struck catches fire unless it saves vs. normal fire, and burning clothing deals 1-6 points per round until put out. A hit on bare skin inflicts 1 point.

Cold-based attacks drop any fyrefly into dormancy at once, as does a dousing with water for an adult; a larva splashed with water must also fail a save vs. paralysis to go dormant. A dormant fyrefly looks dead but revives in 24 hours. Area spells can wipe out most of a larval swarm, but 10% always survive, and the creature the swarm surrounds is usually caught in the blast as well.

Adults live alone in forests, each keeping its own feeding grounds, sleeping by day and eating leaves and rotting plants all night. After two months of constant feeding, a fyrefly seeks out a fire and flies in. Instead of dying, it becomes a brilliant flame-mote, visible as an unusually bright spot in the blaze. If the fire is put out at this point it cannot breed and burns away in 10 minutes. Otherwise, after 10 minutes it splits into two larvae and dies. Each larva doubles every round, and the swarm emerges in waves: 2 larvae in the second round after the first division, then 4, 8, 16, 32 and finally 64, which cannot divide further. Each larva burns until slain or made dormant, or for 10 rounds, then goes dormant by itself and spends 9 days becoming an adult.

A mad wizard bred fyreflies as pets and let them escape by mistake. Only their breeding makes them a menace. They are valued as ingredients in many fire-related potions.