The Crystal Trident is an item found within the Sunken Ship at the bottom of the ocean kingdom, Atlantica. It appears to be a small, blue replica of King Triton's own Trident, made from crystal, without a handle. It is supposed to have the power to reveal Atlantica's Keyhole.
Sometime before Sora and the party arrive at Wonderland, Ariel finds the backboard of the Crystal Trident and adds it to her collection of things from the human world, which is kept at her grotto. Later, when Sora and the party arrive and explore Atlantica, they find the Crystal Trident within a chest in the Sunken Ship, so Ariel reunites it with the backboard at the grotto. Unfortunately, an infuriated King Triton comes in to destroy it, punishing Ariel for disobeying his order to remain the palace and concealing the fact that the world's Keyhole was there.
Following Ursula's defeat, Triton has a change of heart and uses his own Trident to restore the Crystal Trident and reveal the Keyhole so Sora can lock it.
Kingdom Hearts II
The Crystal Trident remains in its base at the back of Ariel's Grotto.