This device is in the shape of a small ball of lead ½ inch in diameter. If the command word is spoken (sometimes etched in tiny printing on the ball itself) and it is placed within the framework of a portal it will explode the next time a creature passes through that portal. The portal need not be active to set the trap. The ball explodes with the force of a fireball cast by a sixth-level caster, causing 6d6 points of fire damage on both sides of the portal, with half the diameter blasting each side and possibly destroying the portal frame. Should such a thing happen, those attempting to pass through the portal emerge from the side they entered.
Though the portal trap may not be disabled, it can be dispelled, and if removed from the portal where it has been planted while the portal is inactive it ceases to function until placed in that same portal.
Caster Level: 6th; Prerequisite: Craft Wondrous Item, fireball; Market Price: 4,000 gp;
Adapted from the Planescape Campaign Setting on Planewalker.com