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". 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, while only Matthew can equip it in Dark Dawn. Its buy value is 17000 coins and its sell value is 12750 coins.

The Gaia Blade increases the wielder's base Attack rating by 135 points, the highest attack rating increase provided by a piece of equipment in Golden Sun, and also increases the wielder's Venus power by 20 and Venus resistance by 20. The sword is a pure-element weapon because 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. 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 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.

In Golden Sun: Dark Dawn
The Gaia Blade returns in Dark Dawn and gains 3 new unleashes: Power Charge, Spiral Assault, and Sky Diver. It is also more powerful than before with an attack power of 168 points. It is found hidden deep within Yamata Ruins (requires the Yellow Orb and Search Psynergy).

Ironically, the Gaia Blade appears to have gotten 2 Unleashes of the Jupiter element which would be considered in conflict with the weapon's original element Venus.

Power Charge resembles releasing an burst of white energy upon hitting the target. Spiral Assault fires twisting wind blades towards the opposition. In the case of Sky Diver, the Adept leaps in the air and swings the blade forward, releasing an golden bird-like spirit that crashes into the enemy with a huge explosion. Finally, the signature Unleash, Titan Blade, summons a massive blade to drop down on its victim and erupts in Venus energy before the screen flashes white.