There are two Tridents associated with Neptune, god of the seas and the Atlanteans.
Trident of Neptune
The original Trident of Neptune was forged by the cyclopes out of admantine and Neptune's own essence. It can manipulate and conjure water, as well as disperse energy in the form of bolts and forcefields. The Trident of Neptune may be used by his chosen champions, and was most recently wielded by Namor the First of Atlantis in Marvel's Universe.
Sacred Trident of Neptune
Approximately 2000 years ago, the Atlanteans were lead astray and started following Set. With his loyal worshipers, Neptune fought Set on Earth and banished the snake god. Neptune then lived among the Atlanteans for a time bringing about a golden age of peace and prosperity. Before Neptune returned to Olympus, he forged a replica of his Trident in the Brazier of Living Flame and left this artifact with his priests for the ruler of the Atlantis.
At some point, this Sacred Trident was lost. When Namor regained his memories and found his people, he was crowned. But his long absence and doubts about his loyalties lead to unrest and rebellion. He embarked on a quest to find the Sacred Trident of Neptune, a relic that would grant him irrefutable validation as ruler of Atlantis.
The Atlantean High Priest of Neptune can call for a Ritual of Renewal, as Kormok did in Sub-Mariner #17. When this happens, the ruler of Atlantis must return to Antarctic Atlantis and dip the Sacred Trident in the Brazier of Living Flame.