Bul-gae

The dark king keeps dogs and sends them up after the sun and the moon, over and over, because he wants light for a country that has none. They always fail. The sun is too hot to hold and the moon is too cold, and the dog comes back with a scorched mouth or a frozen one and is sent out again the next time. What reaches the ground is a fire dog that has been failing for centuries and cannot stop, and it will bite anything that shines.

Bite the Light: it attacks the brightest thing present - a torch, a lantern, a burning weapon, a spellcaster mid-cast - in preference to any other target, always, even at obvious cost. Piercing, melee 5 ft, +4 to hit, 2d6+3 (two bites). Scorched Breath: 15 ft cone, save Dex Quick, fail save roll 2d8+2 fire, cooldown 2 rounds.

Burns Its Own Mouth: every time it bites a source of light or fire it takes 1d6 itself and does it again next round regardless. Douse and Live: a party that puts out every light and fights in the dark is largely ignored, and fighting a fire dog in total darkness is its own problem. Immune to fire, vulnerable to cold.