Master Bolt

"The master bolt, which packs enough power to make mortal hydrogen bombs look like firecrackers."

- Chiron to Percy Jackson

The Master Bolt is the symbol of Zeus' power upon which all other lightning bolts are patterned after.

History
The master bolt is the first and most powerful weapon that the Cyclops made for a god. The purpose of it was to become Zeus' personal weapon (symbol of power) and to help battle the Titans during the First Olympian War. The Master bolt sheared the top off of Mount Etna and hurled Kronos from his throne. It is considered the most powerful weapon in the world. There are many different versions of the Master Bolt in mythology.

The Lightning Thief
Percy Jackson is accused by Zeus of stealing his Master Bolt for Poseidon, as gods cannot steal each other's symbols of power, and Percy is a son of Poseidon and lives in New York City. Because of this, Percy goes on a quest to retrieve the Master Bolt. Luke Castellan is actually the one who stole it, but Ares had caught him soon after the theft. However, Ares didn't turn Luke over and kept the bolt and Hades' helm in order to start a war between the gods. He had also glitched the backpack that was given to Percy so that the bolt would appear inside it when he would arrive in the Underworld, making Percy look like the thief. Percy accuses Hades at first but Hades denies this, saying that his Helm of Darkness was also gone. Percy retrieves both items and returns them safely after fighting Ares.

The Last Olympian
Zeus uses it to attack Typhon, but it just made him stumble backwards, only injuring the beast slightly, disorienting it, and making it more angry.

The Son of Neptune
It's mentioned when Percy sees the statue of Jupiter at his temple. He claims that the Master Bolt doesn't look like that, but Octavian and Hazel Levesque don't understand what he is talking about or what the Master Bolt is.

The Lightning Thief
The Master Bolt is portrayed as a shaft of crystallized lightning that can discharge bolts of electricity. Instead of being hidden in a backpack by Ares, the bolt was concealed in the handle of an expandable shield Luke gave to Percy for his quest.

Description
It is a two-foot-long cylinder of high-grade Celestial Bronze capped on both ends with god-level electrical explosives. It makes mortal hydrogen bombs look like firecrackers according to Chiron. When it is activated the ends of the cylinder emit crystallized electric bolts making it look like a classical lightning bolt.

Sheath
Ares claims that the backpack he gave to Percy was the Master Bolt's sheath, only morphed a little bit to disguise it. It is unknown whether the backpack was an actual sheath that belonged specifically to Zeus. According to Ares, the bolt is magically connected to the backpack/sheath and will always return to it when lost, much like Percy's sword. This heavily implies that the sheath was stolen from Zeus.

Trivia

 * It appears that Jupiter does not have a version of the Master Bolt, because when Percy mentioned it to Hazel she didn't appear to know what he meant. However it is possible that she simply had never heard of it.
 * It Is only rivaled by Poseidon's Trident and Hades' Helm of Darkness.
 * A stylized version of it is the astronomical sign of the planet, Jupiter.