Item from Percy Jackson by Rick Riordan
Percy's sword — a Celestial bronze blade disguised as a ballpoint pen that always returns to his pocket. It kills monsters but passes harmlessly through mortals.
Riptide was a gift from Poseidon, delivered through Chiron in the guise of a ballpoint pen — click the cap and three feet of gleaming Celestial bronze blade extends from nothing. The sword has been wielded by heroes for centuries, passing from hand to hand through the ages, and it chose Percy the way all great weapons choose their wielders. It kills monsters and can hurt gods but passes through mortals like they're not there. If Percy loses it — drops it, throws it, has it knocked from his grip — it reappears in his pocket within minutes, which is either convenient or unsettling depending on your perspective.
In pen form: an ordinary capped ballpoint pen. Uncapped: a three-foot-long leaf-shaped Celestial bronze sword that glows faintly. The blade is inscribed with 'Anaklusmos' (Riptide) in Ancient Greek. It's beautiful, deadly, and fits in a jeans pocket.
Also known as: Riptide, Anaklusmos