When the magic of a portal begins to fail, it might transform into an erratic portal.
Effect: When a creature ends its movement in an erratic portal square, it must make a saving throw. If the creature saves, it can teleport up to 5 squares per tier as a free action. If it fails, the creature can teleport up to 2 squares per tier as a free action, and it takes 10 lightning damage per tier.
Usage: As with all terrain that teleports or moves creatures, use erratic portals to create a complex encounter that allows the combatants to rapidly move around the battlefield. This will make life difficult for defenders, but also rewards those who can make an attack against a marked opponent that shifts away. Include elite or solo monsters (and their better saving throws) to make it a challenging encounter.
Published in Manual of the Planes, page(s) 22.