These rare, glowing rock formations exist in the Underdark and deep caverns. The glow comes from the gas inside the crystals, which explodes if it comes in contact with air.
Effect: A fist-sized doomlight crystal emits dim yellow light in a 3-square radius. If a character in a square with a doomlight crystal is hit by a close or an area attack, the crystal is destroyed as well—and the gas inside it explodes, dealing 5 damage per tier to the character in its square.
Usage: Obviously, this terrain feature makes area and close attacks more powerful. It can also add an interesting lighting element to a battle if the doomlights are the only source of illumination. As they are destroyed, the battlefield grows darker and darker.
Published in Dungeon Master's Guide 2, page(s) 59.