A pressure plate or hair-thin tripwire on the far side of a door triggers deadly frost rays to shoot from the door frame at whatever is passing through it.
The first person to pass through the doorway is safe, but anyone or anything immediately following is blasted with 1d8 cold damage. The pressure plate resets unless disabled, so that each person through the doorway triggers the trap on the person immediately following.
The frost rays are magical, and can be countered by opposing magic.
Black door by Edan Cohen. Unsplash License.