Ares' Sword is a sword and is one of Ares' Symbols of Power.
Percy Jackson and the Olympians
The Lightning Thief
Ares' most likely wielded it against Percy when he defeated the latter in a duel, before Kronos came to the demigod's rescue.
Ares originally disguises it as a hunting knife when he first meets Annabeth Chase, Grover Underwood, and Percy Jackson. He uses it to clean his fingernails, threatening the mortal waitress to take the golden drachma as payment for the burgers that the three demigods eat. 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.
The Titan's Curse
When Ares encounters Percy again, he instantly pushes the point of his sword under Percy's chin in a threatening manner, though he later lowers the weapon, not willing to make himself look unattractive to Aphrodite, who wanted to speak with the demigod.
The Last Olympian
Ares wields his sword while helping the other Olympians battle Typhon, ultimately succeeding in wounding the colossal monster in the nose before Poseidon helps defeat him.
The Heroes of Olympus
The Blood of Olympus
During the Seven Heroes of Olympus's final battle with the Giants in Athens, Ares arrives, helping Frank smash through an entire phalanx of Giants, although Ares is not seen using his sword, instead wielding his spear and disemboweling them "as easily as a child destroying piñatas."
Appearance
Although the sword can morph into whatever weapon Ares wants, such as a shotgun or a baseball bat, its true form is that of a long sword (longer than Riptide) with a skull-shaped pommel and a large ruby embedded in the skull's mouth.
Trivia
- Oddly, Ares' symbol of power in the classic myths was a bloodstained spear which is shown on the banner of the Ares Cabin; it is possible the sword can transform into that.