Icy Terrain Maker (4e Trap)
From D&D Wiki
A magical object taking up one square, such as a a pillar with a orb carved from ice mounted on it, that creates treacherous patches of ice on the ground, hampering its targets. Once the pillar is destroyed the orb falls and breaks into chunks of ice. 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 an icy 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 try to avoid the difficult terrain the trap creates. This would work particually well in combination with monsters that are immune to cold dammage, as it can create difficult terrain around them.