Burning Hands Statue (4e Trap)
From D&D Wiki
A magical object taking up one square, such as a burning statue of a hand, from which a fierce burst of flame can erupt to scorches those nearby. Once it is destroyed it becomes harmless and the flames go out. It attacks all who come within range, unless it has been enchanted to not attack them. It is often used in dungeons or fortresses with a fiery theme. Players can make an Knowlage Arcana Check, DC 10 to identify it when they see it.
This could be used in an encounter by positioning it so that players are likely to walk near it when trying to attack the monsters, making the encounter more difficult as they must avoid the statue. This would work particually well in combination with monsters that are immune to fire dammage, as it can attack while they are within range of it.