Uruna Edge Knife

The Uruna 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 armor of a Knightmare all the way to the cockpit. Developed by the Europia Union, it lacks the oscillating properties of the Britannian Maser Vibration Sword or the Japanese Revolving Blade Sword, but compensates for its unusual design by 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 Series and the Florence.

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

The Uruna Edge Knife is likely called such because its name is similar to 'ulna", and it is stored along where the ulna bone would be in the human body.