Elven Rapier

The Elven Rapier is a Light Blade-class Artifact weapon found in Golden Sun and Golden Sun: Dark Dawn. It is a pure element weapon that is aligned with Jupiter, and is often found in the early portions of each game.

Basic description by game
An Elven Rapier increases the wielder's attack by 44. As a pure-element weapon, its normal attack always counts as an elemental physical attack, one where the damage from the attack is either increased or decreased based on how the user's Jupiter power compares to the target's Jupiter resistance. in the original Golden Sun, it is found in an early section of Bilibin Cave accessible by entering an alternative entrance using the Growth Psynergy. The Elven Rapier holds the distinction of being the only Jupiter-based pure-element weapon in Golden Sun. It can be sold for 1650 coins, and, as an artifact, can be rebought for 2200 coins. The Elven Rapier can be equipped by Isaac, Garet, and Ivan.

The Elven Rapier's Unleash effect is Vorpal Slash (Wind Weasel in Japan), which adds 16 damage points and then modifies the  damage based on Jupiter power and resistance as before. It has the standard base unleashing rate of 35%. Vorpal Slash appears as the screen blacking out at the moment the strike connects to show a giant jagged horizontal burst of green energy (a red palette swap of this effect, sans the black screen, is used as the visual for the Blow Mace unleash).

In Dark Dawn, the Elven Rapier can be found in a treasure chest in the Konpa Ruins. As a light blade, it can be equipped by Matthew, Karis, Tyrell, Amiti, Eoleo, and Himi. Due to revisions to the Unleash system, the Elven Rapier gains a second Unleash, Critical Strike, which multiplies physical damage by 1.5. Due to the Elven Rapier's status as a pure-element weapon, Critical Strike is converted from a non-elemental unleash to a Jupiter-aligned Unleash, and takes into account the wielder's Jupiter Power and the target's Jupiter resistance when calculating damage. Vorpal Slash remains the main unleash but is slightly revised to hit up to three targets at once (the primary target and two enemies next to it).

Analysis
The Elven Rapier being located where it is means it is possible to retrieve it right after the party of  Isaac, Garet, and Ivan first travel through Goma Cave to reach  Bilibin, which is very early in the main quest. At this point, the weapon practically comes across as abnormally powerful, for it is a light blade with more Atk and offensive prowess than the expensive  Broad Swords sold at Bilibin. The portion of the game taking place at Kolima Forest and Tret Tree would be quite easier and quicker  if the player goes to Bilibin Cave, takes the Elven Rapier, and levels  up with the stronger monsters at the cave before heading back down to  Bilibin and continuing the quest's "normal" path.

As a weapon, the Elven Rapier is also noteworthy for being a pure Jupiter-element weapon whose normal attacks register as Jupiter-element attacks in addition to its Jupiter-based unleash, and this elemental affinity affects its damage based on the Jupiter Resistance that each enemy has. Meaning, it deals much enhanced damage against monsters with very little Jupiter resistance, namely all the monsters in the Bat, Amaze, Ghost, and Drone Bee monster lines. This strong weapon will likely be in the party's possession for quite a while, and is good to equip on Isaac who normally has the highest attack (at least as long as he's made an Illusionist, in which case he'll have more Jupiter power to empower the Rapier), and when he gets a stronger weapon (like the Arctic Blade) it's a great hand-me-down weapon to equip on Ivan.

In Dark Dawn, the Elven Rapier is an significant upgrade over any weapon available up to this point. As a Jupiter based weapon, it will make an excellent upgrade for Karis from the Magic Rod she likely possesses, and due to its high attack increase, will remain in her possession for some time. By this time, Matthew will already possess the Venus-based Themis' Axe, while Tyrell can later acquire the Mars-based Blow Mace. The revisions to Vorpal Slash essentially give Karis a ranged elemental physical attack, while Critical Strike, due to its damage multiplier, can grow stronger as Karis's attack increases. Thus, both unleashes can be very useful.

Trivia

 * The name of the Unleash effect of this blade is a rather common RPG reference to the sword used by the protagonist of Lewis Carroll's poem, "Jabberwocky".
 * Aside from this weapon and the Elven Shirt, Elves do not appear in the Golden Sun series in any way.