Also called Anaklusmos (literal meaning: the sudden strong current that pulls sailors off course at sea), this is Percy Jackson's celestial bronze sword. It is a ballpoint pen that turns into a sword

Percy and Riptide

when uncapped. If he loses it, it returns to his pocket soon, which is very useful, meaning he can never lose it (unless he isn't wearing any pockets) then it would just go to his hand.

This is a very old weapon once used by Heracles, but it is still very effective. Percy is very skilled with it and has killed many monsters with this weapon, like Dracanae for example.