The Trident was a prominent artifact in Disney's "Little Mermaid" series and film, it granted the user complete command over the forces of the ocean as well as formidable size-shifting abilities and (perhaps as an extension of its control over the ocean) the ability to conjure storms.
The Trident is traditionally used only by King Triton due to his rightful place as king of the seas (a parallel both to ancient Greek/Roman myth, in which the god of the seas had a trident, and Arthurian legend in which the rightful ruler would have a unique and powerful weapon).
However it was also used unlawfully by Ursula in a failed attempt to conquer the oceans - while under Ursula's command the trident was shown to be able to unleash destructive blasts of energy and could transform people into pathetic worm-like creatures or even transmute them into small plants (presumably plankton or similiar).
It is unknown how much of these traits were the Trident's or Ursula's own witchcraft but the destructive energy beams were also seen to be used by King Triton when he destroyed Ariel's beloved treasures from the human world in a fit of rage (which he later regretted).