Pits, electrifying runes, lava, extremely deep water, and other harmful environmental phenomena are hindering terrain, which punishes creatures that are in it or try to enter it.
Saving Throw: A creature can make a saving throw to avoid being forced into hindering terrain, whether it is pulled, pushed, slid, teleported, or otherwise moved against its will. See “Forced Movement” and “Teleportation".
Damage: Hindering terrain almost always has the potential to harm creatures that enter it, either by causing them to fall or by dealing damage to them directly. (Some hindering terrain might impose a penalty or a harmful condition without dealing damage.)
Published in Rules Compendium, page(s) 312, Monster Vault: Threats to the Nentir Vale, page(s) 123.