Phasing

While phasing, a creature ignores difficult terrain and can move through obstacles and other creatures, but it must end its movement in an unoccupied space.

Published in Monster Manual 2, page(s) 218, Monster Manual, page(s) 282, Player's Handbook 3, page(s) 222, Rules Compendium, page(s) 314, Monster Vault: Threats to the Nentir Vale, page(s) 124.