Poseidon's Trident

The Trident is Poseidon's weapon and his symbol of Power. With it, he has the ability to control the ocean. It can also project beams of blue light from it that can transform targets or even outright destroy what they hit. It is shown that when he hits a sea monster, it transforms into a goldfish. When he hit a squid "it dissolved like food coloring", and when he told Percy, "'If I hadn't looked before I blasted, you would be a puddle of sea water.'" It is also said that earthquakes are caused when Poseidon strikes his trident into the ground. That is why Poseidon is called "The Earthshaker." It was forged by sea demons known as Telekhines. It is a three pronged spear with energy flickering around it. To mortals, it generally appears as a surfboard or a fishing rod when he is by the beach due to the Mist. There is an attachment for Poseidon's trident on his throne, where one would expect a fishing pole on a mortal fisherman's chair.

The Lightning Thief (film)
Poseidon first appears with his trident when he walks out of the ocean. Percy uses one made from water to defeat Luke in The Lightning Thief movie.

The Titan's Curse
At the Council of the Gods on Winter Solstice, Poseidon sides with his son Percy's judgment. He declares that he wouldn't allow the death of the Ophiotaurus, his trident appearing in his hand when he was angered.

The Last Olympian
Poseidon uses his trident against Typhon, throwing it into Typhon's throat so golden ichor spewed out like a waterfall as tall as a skyscraper.

Trivia

 * The trident is the astronomical symbol of the planet Neptune.
 * In myth, the most common origin for the trident is it was created by the Elder Cyclopes along with Zeus's lightning bolt and Hades's helmet. The books use the less common story of the Telekhines forging it.