Percy Jackson’s magical sword in Rick Riordan’s Percy Jackson novels is called Riptide.
Riptide is the English name used for Percy’s sword, whose Greek name is Anaklusmos. The weapon is made from celestial bronze, a material that can harm monsters and gods but normally passes harmlessly through ordinary mortals. Percy carries it in a disguised form: it appears as a ballpoint pen when not being used as a sword.
The pen returns to Percy if he loses it, making it both a weapon and a recurring magical device. Chiron gives it to Percy early in the series, and its connection to the world of Greek mythology helps reveal Percy’s identity as a demigod.
A common mix-up is treating Anaklusmos as a different sword. It is the Greek name for the same weapon, while Riptide is the name most English-language readers encounter. Backbiter is Luke Castellan’s sword, not Percy’s.