Forcecage Trap (4e Trap)
From D&D Wiki
A innocent looking tile on the floor conceals a secret - when stepped upon, it erects an invisible cage made of unbreakable bars of force, imprisoning the creature upon it. The creature trapped can attempt to deactivate the Forcecage, but at a -10 penalty as they are cannot move very much. When it is detected or it activates, players can make a Knowlage: Arcana check to find out what it is.
Forcecage traps can make encounters more difficult by removing one or more of the adventurers from the battle. The remaining players face the choice - help them or continue the fight without them?