This is a sword from Percy Jackson and the Olympians book series
Wielded by Percy Jackson himself

Its name is Anaklusmos, and when translated from Greek it is called Riptide
The dormant form of the weapon is a pen, and when uncapped, it becomes a sword made of celestial bronze, able to kill monsters and halfbloods, but not humans

I made this based off of a picture from the wiki it is (supposedly) from the first graphic novel