The Trident is a magical weapon that is featured in The Little Mermaid franchise. It serves as the personal weapon of King Triton, and only Triton or members of his bloodline can remove it from its resting place. It has vast magical abilities, and thus has been strongly desired by both of the sea witches Ursula and Morgana.
The trident is shown to have various applications, depending on the will of the user. When its power is activated, it illuminates with a bright, shining golden or blue glow (green in Morgana's case) and is heard to make a deep humming sound punctuated with high-pitched melodious tunes. Since whoever wields it has command over the oceans, it affords tremendous power.
Most commonly seen is its ability to fire destructive energy bolts. These energy bolts, however, must be fired immediately upon the ready, or they will cause the built up energy to backfire on the user. This is ultimately how Ursula was killed, as she attempted to fire an energy ray at Ariel that was implied to be unavoidable, but was impaled seconds before she could fire by prince Eric and one of the ships, causing the energy to backfire on her. Though these energy bolts may destroy many things, the trident is incapable of destroying or undoing Ursula's deal with Ariel.
Triton has also used it to summon rain storms, and transform his daughter, Ariel, into a human. He is also seen creating a rainbow in the first film, and disintegrating the sea wall in the second film. Ariel uses it herself in one episode of the television show, to thaw dinosaurs that have been trapped in a glacier, and it is revealed that the trident is activated simply by the will of the user. When the trident is in Ursula's possession, she grows to gigantic size and creates a giant whirlpool in the ocean that has such force that it raises sunken ships from the ocean floor (which Eric uses to kill Ursula). In the television series, the trident can be mentally controlled at Triton's whim and called to him as well.
In The Little Mermaid II: Return to the Sea, it is revealed that the trident's powers have no effect on pure-blooded humans (or human-merpeople hybrids in Melody's case); therefore even if Ursula did get a clean shot on Eric, when she attempted to kill him in the first film, it would not have any effect on the latter.
Powers and abilities
Energy Bolts: The trident can generate, create and project powerful bright gold, pink, green or blue energy bolts; that can destroy or disintegrate almost any substance known to humans and merpeople.
Size Manipulation: The trident can increase or decrease the size of the user, other merpeople, aquatic/marine creatures, etc.; while preserving body proportion.
Conjuring: The trident can generate, create and project a rainbow.
Shapeshifting: The trident can transform its user or other merpeople into humans and vice versa.
Levitation: The trident can levitate other merpeople, objects, aquatic/marine creatures, etc.
Corporikinesis: The trident can manipulate the bodies of other merpeople or aquatic/marine creatures; forcing them to do the user's bidding against their free will.
Immobilization: The trident can temporarily incapacitate other merpeople, aquatic/marine creatures, etc.
Atmokinesis: The trident can generate, create, and manipulate different aspects of the weather; such as rainstorms, thunderstorms, etc.
Cryokinesis: The trident can generate, create, and manipulate ice.
It should be noted that despite the claim that only members of the royal bloodline can remove the trident, others have been able to; for instance, Urchin in the TV series was able to lift the Trident out of its resting place at one point despite him not being related to Triton at all. It can even be used against the Royal Family of Atlantica, seen when Ursula misuses its powerful lightning bolts (meant to defend Atlantica) to instead try to murder Princess Ariel.
However, considering the claim was made in Return to the Sea, which took place after the events of the original film, that may have implied that King Triton made that requirement of only members of the royal bloodline could remove the trident to avoid a repeat of the events of the movie.
In A Twisted Tale: Part of Your World, when Ariel becomes Queen after Triton is captured by Ursula, she is able to turn the trident into a comb when she assumes human form to search for her father. It is also established that the trident only has power when in the sea or in contact with sea water, with the result that Ariel cannot use its powers when on land (although she is able to channel its power when she makes contact with a fountain that takes its water directly from the sea).