Ruby's Demon Killing Knife – Supernatural. Overall Length 15cm. Blade material is Stainless Steel. Survival Knifemini Replica. The Demon-Killing Knife, or most commonly known as Ruby's Knife, is an ancient knife that was created by the Kurds which possesses the power to permanently kill most demons, hellhounds, and other demonic entities. Though it somehow came into the possession of Ruby, it eventually passes onto Sam and Dean Winchester, who use it as their most common means of killing demons.This is the Mini Replica as aletter opener and prop display.