The Trident is the godly weapon of Poseidon, god of the seas. This powerful weapon allows him to control the seas and exert vast magical influence over water in all of its forms.
The trident grants whomever possess it control over the oceans, rivers and water in all of its forms. It's possible to shift water into its various forms such liquid, gas, solid and create atmospheric hazards such as storms and vortexes. Also the trident is powerful enough to pierce a god's skin.
Although the trident's actual manufacturer is unknown, the trident is Poseidon's iconic weapon for eons until it was stolen by Hades in his plans to take over Athens by shifting river Styx. Hercules later returns the trident to its rightful owner and the only time it shifted ownership was when Triton, Poseidon's son, was in temporary ownership of the trident.
In the series, the Trident belonged to Ursula's father Poseidon. The King of the Seas, he used it to rule over his domain. When discovering that his daughter wanted to run away with the aid of one Captain Hook, he used his trident to enchant a sea-shell which would take away her singing voice and prevent her from leaving him. After Ursula learns of his betrayal, she takes his trident and gives herself more power, consequently turning her mermaid tail into those of an octopus.