In some areas, lightning energy fuses with rock to form a highly unstable, dangerous mixture. In various regions of the Elemental Chaos, and in dungeons located near the conflux of different types of elemental energy, blue-tinged areas of bolt stone appear.
Effect: When a creature enters a square of bolt stone, it takes 5 lightning damage per tier. Roll a d20; on a 10 or higher, the bolt stone’s energy is discharged, and the stone becomes inert. Otherwise, the lightning energy is transferred to all unoccupied squares adjacent to the origin square, and those squares become bolt stone.
Usage: Bolt stone works best when you scatter it across an encounter area. It offers some risk and reward. A player character might enter a square and take the damage in the hope of creating a barrier to hold back monsters or eliminate minions.
Published in Dungeon Master's Guide 2, page(s) 58, Dungeon Master's Kit, page(s) 209.