Mystery Blade

The Mystery Blade is a Light Blade-class Artifact found in Golden Sun and Golden Sun: Dark Dawn.

Basic description by game
The Mystery Blade increases the wielder's attack by 84 points. It can sold for 4800 coins, and can be bought for 6400 coins. In Golden Sun, the Mystery Blade is found on the second floor of Crossbone Isle, and can be equipped by Isaac, Garet, and Ivan. The Mystery Blade's Unleashed power is Life Nourish. Life Nourish converts all damage to Venus damage and attacks with an additional 25 damage points. Also, it has an excellent chance to heal the attacker by an amount equal to the damage dealt. Life Nourish appears as a swarm of purple motes of energy being pulled from the target to the attacker at the moment of contact.

In Dark Dawn, the Mystery Blade can be found in a chest on the third basement level in the Harapa Ruins, but is only accessible after the player has defeated the Ice Queen. Besides its signature Life Nourish unleash, it gains the Spin Strike unleash. Spin Strike converts all damage to Jupiter damage, and is one of a few unleashes to have a range of 3 targets. As a Light Blade, it can be equipped by Matthew, Tyrell, Karis, Amiti, Eoleo, and Himi.

Analysis
While probably more powerful than anything found up to the earliest point Crossbone Isle can be accessed, Mystery Blade  suffers  because that point is right before arrival at Tolbi. In other words,  the Assassin Blade, a stronger weapon with an  instant-kill Unleashed power of the same element, is very likely to be  gained shortly thereafter. While Life Nourish's Drain effect occurs far more  frequently than Mortal Danger's Condemn effect, the higher  attack  bonuses of both the base weapon and Unleashed power are likely  on their own to out-prioritize the Mystery Blade. However, if one acquires  Hyper Boots at Tolbi, increasing the Unleash rate of the  user's weapons, the more reliable effect of the Mystery Blade becomes a  bit  more desirable, making it possible that the weapon will remain in   Isaac's possession for a fair portion of Gondowan.

In Dark Dawn the Mystery Blade will outpower any other weapon available to that point. The Venus-based Life Nourish unleash makes it a good weapon for Matthew, while the Jupiter based nature of Spin Strike means that Karis could make use of the Mystery Blade as well. It is worth noting that the Grievous Mace, a Venus based pure element weapon, will be available in the next major dungeon, and would most likely make a better choice for Matthew.