Bronllafyn
A knife carried by Osla Big Knife, which was so large that it could be used as a bridge for armies to cross bodies of water. However, it proved to be Osla’s doom when, during the hunt for Twrch Trwyth, the sheath filled with water and dragged Osla to the bottom of a river.
Source
Culhwch and Olwen | Late 11th century