These crystalline shards of solidified thunder energy are both unstable and dangerous. They rumble and shudder as they course with the energy within them. Most creatures avoid them, but occasionally orcs, ogres, and other battle hungry creatures dwell near them. Although thunder shards are dangerous, they also make useful defensive fortifications if used properly.
Effect: Squares of thunder shards are difficult terrain. If a creature makes a melee attack against a target in a thunder shard square, roll a d20; on a 10 or higher, both the attacker and the target take 5 thunder damage and are pushed 1d4 squares from each other.
Usage: Thunder shards are useful barriers to encourage movement and add a random element to the fight. The random push is especially useful if the area has pits or other dangerous terrain.
Published in Dungeon Master's Guide 2, page(s) 61.