Gaia Blade

The Gaia Blade (Gaia no Tsurugi/"Sword of Gaia" in the Japanese version) is a Long Sword-class weapon Artifact available in Golden Sun and Golden Sun: Dark Dawn, and in the former game many would instinctively identify it as the game's "ultimate weapon".

Basic Description by Game
In Golden Sun, the Gaia Blade increases the wielder's base attack by 135 points. In Dark Dawn, the Gaia Blade instead gives a higher Attack bonus, increasing the wielder's attack by 168 points. This makes the Gaia Blade one of the few returning weapons that has its attack boost changed between games. In all appearances, it increases the wielder's Venus Power and Venus Resistance by 20 points. It can be sold for 12750 coins and subsequently rebought for 17000 coins. Additionally, the Gaia Blade is a pure-element weapon, aligned with Venus. its physical attack is itself an attack whose resulting damage is either increased or decreased based on how the user's Venus power compares to the target's Venus resistance.

In Golden Sun, it is found in a treasure chest pretty easily in the game's final dungeon, Venus Lighthouse. Being a Long Sword-class weapon, the Gaia Blade can be equipped on warrior-style Adepts, namely Isaac and Garet, and Felix, and Piers if transferred into The Lost Age. The Gaia Blade's Unleash effect is Titan Blade (Titanic in the Japanese version), with the standard base activation rate of 35%, which is a physical attack with an additional 70 points of damage added, and then is affected by the user and target's Venus statistics as before. Visually, Titan Blade is the most spectacular Unleash effect in Golden Sun, which resembles the wielder summoning a colossal yellow sword crashing down into the area where the target is, letting loose a veritable water spout of yellow energy as it digs into the target, and detonating in a large bright explosion of purple energy.

In Dark Dawn, the Gaia Blade is found hidden deep within Yamata Ruins, and requires both the Yellow Orb and the Utility Psynergy Search to be found. Similarly to the stronger Sol Blade, only Matthew can equip the Gaia Blade. The Gaia Blade gains 3 new unleashes: Power Charge, Spiral Assault, and Sky Diver.

Ironically, none of the Gaia Blade's new Unleashes appear to be of the Venus element: Two of them are aligned with Jupiter, the element in conflict, while the remaining one is with the symbiotic element (Mars). This is a somewhat awkward combination considering the Gaia Blade's Venus elemental nature. Visually regarding its special abilites, Power Charge resembles a burst of white energy being emitted upon hitting the target. The Spiral Assault casts a whorl of wind blades towards the  opposition, dealing Jupiter-based damage to the target and the two  adjacent units. In the case of Sky Diver, the Adept leaps high into the air and swings the blade forward to release a golden bird-like  spirit that dives into the enemy with a huge explosion, which has a  chance to paralyze the target. This sword's signature Unleash, Titan Blade, summons a massive blade to the target's location and erupts in  Venus energy as it digs in the earth before the screen flashes white. It is very similar to its previous visualization but rendered this time.

Golden Sun
In Golden Sun, the Gaia Blade is perhaps quite clearly the best weapon in the game. Most players would equip it on Isaac because when he has all seven Venus Djinn on him his very high Venus power and resistance (further bolstered by the sword itself) causes his normal attack, Unleash, and even his Psynergy moves to be noticeably more powerful than those of any other Adept. A typical method of capitalizing on Isaac's efficiency with the Gaia Blade is to equip the Warrior's Helm, which increases his Unleash rate by 10% and Venus power by 10, and Hyper Boots, which further increases his Unleash rate by 12%. He will be able to Unleash a powerful Titan Blade effect 57% of the time when he attacks, and if he is wearing Spiked Armor that chance even further increases by 6% to form the maximum possible Unleash rating of 63%. (Titan Blade's effect is essentially a more damaging version of the Ragnarok Psynergy attack, which is viewable as a more damaging version of a normal physical attack with the Gaia Blade.)

Considering how powerful and easy to get the Gaia Blade is, it can safely be assumed that if Golden Sun: The Lost Age is played with full data transfer Isaac will be equipped with the Gaia Blade when his party joins Felix's. At that point, even though the Gaia Blade has technically less of an attack rating than weapons Felix's party can have (Hestia Blades and the Phaeton's Blade, namely), the Gaia Blade is still a good weapon for Isaac to wield and can only be reliably replaced by a stronger Venus-based weapon, such as the three possible weapons that can be forged from Golem Cores. One of those three weapons, a Huge Sword, is a direct improvement over the Gaia Blade that shares its pure-element Venus nature and has higher Attack and an Unleash with an additional side effect that lowers enemy Defense, though it does not increase Venus power and resistance.

Dark Dawn
In Dark Dawn, it is worthwhile to get both the Search Psynergy and the Yellow Orb as quickly as possible, so that the Gaia Blade can be gained as quickly as possible. The Gaia Blade will be a significant upgrade from whatever weapon Matthew happens to have at the time, and will remain in his possession until he gains the Sol Blade at Apollo Sanctum.

The power of the Gaia Blade is somewhat decreased due to the increase in potential unleashes. While there is equipment that can increase the Unleash rate (this equipment is generally used on Matthew for a maximum Unleash rate Sol Blade set up), there is no way to guarantee that a specific Unleash will be chosen. However, the three weaker unleashes can be useful in various situations. Though Power Charge and Spiral Assault are fairly weak unleashes, Power Charge can ignore 50% of the targets defense and spiral assault can affect multiple targets, which is useful in random encounters. Though Sky Diver is a Jupiter-based Unleash, it is close in power to Titan Blade and has the useful side effect to stun opponents.

If the player wants to take full advantage of all the Unleashes, they may consider placing Matthew in either the Ninja class or the Ronin class (which one would depend on the user's preference). Both classes increase Matthew's Fire Power and Jupiter Power. His Venus Power is decreased compared to his base class (assuming he had the maximum available Venus Djinn equipped), but the Gaia Blade's Venus Power bonus will help to mitigate that. The player can also equip the Warrior's Helm to further increase Matthew's Venus Power by 10 points.