Uruna Edge Knife

The Ulna Edge Knife (ウルナエッジ, Urunaejji) is a retractable hidden blade concealed in the arm shaft of a Knightmare Frame that is capable of piercing through the metal a Knightmare frame all the way to the cockpit. Developed by the Europia United, it lacks the oscillating properties of the Britannian Maser Vibration Sword or the Japanese Revolving Blade Sword, but compensates for its unusual design making an attack involving it unexpected and thus difficult to guard against. Currently the only Knightmare Frame that is known to field this technology is the Alexander Units and the Florence.

Trivia
The Ulna Edge Knife is similar to the iconic Hidden Blade of the Assassin's Creed video games.

The Ulna Edge Knife is likely called such because it is stored along where the ulna bone would be in the human body.