1 Treacherous Ice Sheet (200xp)
12 kobold minions (300xp)
Hallway leading to a 3x3 room with a treacherous ice sheet in it. Two long hallways branch off from the ice sheet room, perpendicular to the entry hall, each with 6 kobold minions and a barrel of javelins. One passageway contains a lever which raises and lowers a portcullis in the entry hallway.
Once at least 3 PCs have entered the ice room, the kobolds will pull the lever, lowering the portcullis over 1 round, and attack from the darkness with javelins.
Probably not a guaranteed TPK, but it would be crazy peril for most 1st-4th level parties, and it follows the encounter rules. Probably a good encounter for a keep that has a young white dragon lairing in it with a brood of kobold servants, provided your players like it rough.