The Crossbow door trap, it's activated by opening a door. The trap is undetectable unless you have some sort of foresight or can see the future. When the door is opened, a Crossbow fires a bolt, the area it fires at can be determined while it's being set up. Multiple Crossbows can be set up so the victim is hailed by a barrage of Crossbow Bolts. The Bolts can also be poisoned so watch out! Some versions of the trap are made to be obvious and dodged so it can hit and activate a pressure plate trap. Best used in small corridors with multiple poisoned tipped Crossbows, making the barrage nearly impossible to dodge. Almost guaranteed to bring down the best adventurers, particularly rogues and mages for they don't wear much armor.
Crossbow Door Trap
The classical Crossbow Door trap