Ares' Sword

Ares' Sword is the war god Ares' symbol of power.

The Lightning Thief
Ares originally disguises it as a hunting knife when he first meets Annabeth, Grover, and Percy. He uses it to clean his fingernails, threatening the waiter to take the golden drachma. He also cloaks it as an aluminum baseball bat shortly before dueling against Percy. In addition, it is seen by mortals through the mist as a shotgun.

Appearance
Although the sword can morph into whatever weapon its master wants, its true form is that of a long sword with a skull-shaped pommel and a ruby embedded in the mouth.

Trivia

 * Oddly, Ares' symbol of power in the classic myths was a bloodstained spear which is shown on the banner of Ares' Cabin.