These weird crystals collect and store energy of all kinds.
Effect: Energy crystals are difficult terrain and have the following stats: 20 hit points per tier; AC 5, Fortitude 10.
When an energy crystal is hit by a cold, fire, lightning, necrotic, radiant, or thunder attack, it absorbs the energy and takes no damage from the attack. In addition, until the end of the attacker’s next turn, any creature that enters the energy crystal’s square or starts its turn there takes 5 damage per tier of the type absorbed by the crystal. If an energy crystal is destroyed while it holds absorbed energy, it explodes, dealing 10 damage per tier of the absorbed energy type to each creature within 3 squares of it.
Usage: Use energy crystals with creatures that have burst or blast attacks with a damage type that the crystals will absorb and resistances of the appropriate damage type. After priming the crystals with an attack, have those creatures destroy the crystals to deal extra damage in a large burst. Minions also work well at destroying crystals that have been primed, even if they destroy themselves in the process.
Published in Manual of the Planes, page(s) 22.