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 function of the Gaia Blade is changed somewhat with the addition of three other, similarly powerful unleashes. The first two, Power Charge and Spiral Assault, have the relatively low damage multipliers of 1.4 and 1.3, respectively, but they are solid enough damage multipliers nonetheless. Matthew, the only Adept that can equip this sword, is naturally going to have the highest attack rating of the party, and being found very late in the game means the Gaia Blade will be acquired at a point when Matthew's attack rating has been boosted to very high ratings by his level (around the late 500's). Therefore, Power Charge will likely already be around the level of power as Titan Blade, and will only become stronger than Titan Blade as party levels climb in the endgame. None of this is counting the beneficial side effect Power Charge has to sharply increase its damage by ignoring half of the enemy's defense. Even Spiral Assault can prove itself more useful than Titan Blade because it hits multiple enemies for close to the same amount of damage as Power Charge. On its own, even Sky Diver is better than Titan Blade, having an almost identical damage bonus and the potential added stun effect. However, it must be noted that Titan Blade has an inherent advantage in being the Gaia Blade's only Venus-element unleash. Matthew naturally has a high Venus power, which is emphasized even more by the Gaia Blade, but the difference shrinks if Matthew is in a class series such as Apprentice or Ninja which give him almost as high Mars and/or Jupiter power. This would be a good decision anyway to take full advantage of all four of the Gaia Blade's unleashes. Therefore, depending on Matthew's class and level, any of the Gaia Blade's unleashes can be better than any other.