The Shadow's Blade

The Shadow's Blade is a short sword made for backstabbing and thus is made for assassins. The assassin Slythe drops this item as loot when he is killed during his failed attempt at killing the protagonist.

In-game description
"Short sword of backstabbing: 'The Shadow's Blade.' The Shadow's Blade was created to be the perfect assassin's tool. It is highly sought after by any who settle differences with a blade, and many that possess it do not do so for long."