Arc Qui Ne Faut, Fail-not
A keen booby-trap invented by Tristan.
While Tristan and Isolde were forced to hide in the forest of Morrois to escape Mark’s wrath, Tristan set up the Unfailing Bow to kill any animal or person that happened by – partially to procure food and partially for protection.
Source
Tristan | Béroul, late 12th century