Door Axe (3.5e Equipment)
From D&D Wiki
A large axe is attached to the top of the door frame, either held against the wall above the door, or attached to the ceiling. When the door is opened a thin wire attached pulls the axe which swings right through the open door. Damage varies based on the qualities of the axe. A Reflex save allows the character to avoid the cut, provided his friends aren't all rushing in after him. An alert character can see the wire and bypass harm.