The riptide pen is the sword of the main character, Percy Jackson, in the "Percy Jackson" series. The sword is made of a special material that can kill gods, demigods and other mythological creatures.
In the story, the sword draws its power from water, which is why it is attracted to Percy. It was once owned and used by Hercules, before Percy. When dormant, the sword takes the form of a pen, hence its nickname. The sword is made from a material known as Celestial Bronze and contains an enchantment so it will magically appear in Percy's pocket if lost.