Force Orb Blaster (4e Trap)
From D&D Wiki
A magical object taking up one square, such as a ornate cannon on a pillar, that blasts orbs of magical force at its targets, that burst and throw off razor-sharp shards of force that cut nearby creatures to ribbons. Once the pillar is destroyed the cannon falls and breaks into worthless shards of metal. It attacks all who come within range, unless it has been enchanted to not attack them. It is often used as a protection in dungeons and fortresses that have a technological or force damage 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 force damage, as it can attack while they are adjacent to the target, as they will be immune to the secondary attack.