Target: one creature
Attack: Intelligence vs. Reflex
Hit: "3d10 + Intelligence modifier force damage. The target is confined in the forcecage until the end of your next turn. While confined, it is immobilized, grants combat advantage, and cannot gain line of effect against nonadjacent enemies."[PH:168]
Miss: "Half damage, and the target is slowed until the end of your next turn."[Dr401:73][U :12/2011]
Forcecage is an encounter power available to wizards at 27th level. It is a powerful immobilization spell that affects one target.
The "Class Compendium: The Arcanist" article in Dragon 401 added the evocation keyword to forcecage, and added half damage and slow condition on a miss. Previously, forcecage had no effect on a miss. The December 2011 update finalized these changes.