Trident of Ankohl

The Trident of Ankohl (as an item, it is referred to simply as the Trident is a truly unique piece of equipment found in Golden Sun: The Lost Age. It is needed to break Poseidon's protective barrier and allow Felix's party to battle the entity outright. It must be forged from its three prongs, which are convienently named the Left, Center, and Right Prongs.

The Trident of Ankohl can be equipped (overlapping nothing else) to raise the wielder's Mercury Resistance by 20. When used in battle, the Trident attacks with 120% the user's attack rating as well as converting all damage to Jupiter damage. When battling Poseidon, the Trident destroys Poseidon's force field the first time it is used. Unless the force field is destroyed, Poseidon will be invulnerable to all attacks. It should be noted that the Trident does not need to be equipped in order to be used for attacking, although it also doesn't take up an equipment slot when equipped.

Interestingly, an old man in Madra seems to know about the Trident, and how it's necessary to defeat Poseidon and get to Lemuria.

Forging the Trident
In order to create (or perhaps recreate) the Trident of Ankohl, the player must first complete Briggs' subplot. Afterwards, speaking to Obaba with any of the three pieces of the Trident will encourage her to help Felix put the pieces back together. In fact, she'll even hold on to any pieces the player has collected, freeing up valuable inventory space. After all three pieces have been given to Obaba, she'll throw them into her forge and the Trident of Ankohl will pop out...and float in midair. At this point, retrieving the Trident is as simple as using Reveal.

The hard part is actually getting the prongs.

Right Prong
The Right Prong is the first piece of the Trident available to players, and may be the easiest to obtain. It is located in the Shrine of the Sea God, which many players will probably have already explored to get their hands on the Jupiter Djinni Breath. Although most of the shrine can be explored with the Lash Psynergy, the Right Prong remains beyond the player's range until he/she returns from Daila with the Sea God's Tear. Using the Sea God's Tear on the large lady-like statue in the shrine will cause the water level to rise (the platform the statue is on rises along with it), giving players access to the prong.

Center Prong
The Center Prong can be found in Tundaria Tower. Perhaps somewhat mockingly, the prong is located near the entrance, almost within the player's grasp but still just oh-so-far out of reach. The prong is sealed within a block of ice which can only be destroyed with the Burst Psynergy. Fortunately, the Burst Brooch can be found at the top of the tower, letting players kill two birds with one stone. The bad news is that scaling the tower is impossible without the Parch Psynergy, so Aqua Rock must be cleared before either the prong or the brooch can be obtained.

Left Prong
The Left Prong can be found a lot closer to home than the other two. This piece of the Trident of Ankohl is resting in the Ankohl Ruins, just east of Champa. It can be acquired with a little help from the Sand Psynergy, which can be learned at the (relatively) nearby Gaia Rock.

In Dark Dawn
The search, forging, and subsequent usage of the Trident of Ankohl is referenced several times in Golden Sun: Dark Dawn, and has apparently become general knowledge. The first mention is in the Psynergy Training Grounds, a theme park dedicated to the now-legendary Warriors of Vale. The search of the Trident is featured in the Jupiter sub-area as a series of three Move puzzles, which uncover prongs of the Trident. After all three puzzles are completed, the geysers blocking a wooden replica of Poseidon disappear, in a nod to the original Trident's ability to destroy Poseidon's force-field. The Trident is also mentioned in the fourth book of the Sun Saga, a series of books detailing the journey of the Warriors of Vale. Here, the implication is given that Felix found the Trident in one piece in the Ankohl ruins, though this may be simply due to the books' compacted nature.

Trivia

 * The Trident of Ankohl is the only usable equipment that cannot break.