Kikuichimonji

A Kikuichimonji is a Light Blade found in Golden Sun. It is dropped by Fenrirs in Venus Lighthouse, although the odds of receiving one are very slim. Being a Light Blade, it can be equipped by Isaac, Felix, Garet, Jenna, Ivan, and Piers. It increases the wielder’s attack by 128, making it both the most powerful Light Blade and second most powerful weapon in Golden Sun. It can be bought for 13400 coins and sold for 10050 coins.

The Kikuichimonji's Unleashed power is Asura (Shura no Mai/Dance of Ashura in Japan). It converts all damage to Jupiter damage and attacks with an additional 64 attack points. It also has a chance of ignoring 50% of the target's defense, increasing the amount of damage done. Asura appears as a samurai with a tengu mask performing a horizontal cut on the target at the same time as the wielder.

A Kikuichimonji is a rare but powerful weapon with few rivals to speak of, outdone only by the Gaia Blade in terms of attack power. However, there are other weapons that can challenge its position, most notably the Swift Sword. Although physically weaker, the Swift Sword's Unleash, Sonic Smash, occasionally triples the damage done, which can overpower even the Gaia Blade. Sonic Smash also does Jupiter damage, and the Swift Sword increases the wielder’s Jupiter power, making it seemingly the perfect weapon for Ivan. However, the chance that the unleash does triple damage is not very high, and the attack bonus of the Kikuichimonji is far greater than that of the Swift Sword and the Kuikichimonji's unleash is also strong (unleash does Jupiter damage and has a chance to ignore 50% of the foes defense), so it's really a matter of preference whether you want to keep the Swift sword or give him a Kikuichimonji. However, all these weapons are found late in Golden Sun and can easily be outdone in Golden Sun: The Lost Age, so their usefulness is somewhat limited.

RNG Methods

 * Main article: Random Number Generator

This is a method for obtaining the Kikuichimonji. Make sure Isaac is faster than Mia and Garet is slower than Mia but at least around 150. You can do this by equiping Running Shirts, Ninja Garb, Ninja Hood, Elven Shirt, Kimono, or Feathered Robe, and if any of them is too fast, you can slow them down by equipping the turtle boots. Go to the main room in Venus Lighthouse (the one with the Psynergy Stone in the middle). Save and turn off the GBA completely. Turn it on again and run around the room to engage in a battle. It should be against an Ice Gargoyle and a Fenrir and you should have the first strike.

First Round (big arrow on Fenrir) :

Isaac casts Clay Spire.

Garet casts Flare Wall.

Ivan casts Tornado.

Mia casts Ice Horn.

Second Round:

Isaac defend

Garet unleashes Torch (Works with Scorch as well) on Fenrir.

Ivan defend

Mia casts Wish.

Third Round:

Isaac unleashes Bane on Ice Gargoyle

Defend with everyone else

This should drop the Kikuichimonji and a Potion. This method can be repeated to your heart's desire.

In Dark Dawn
The Kikuichimonji returns in Golden Sun: Dark Dawn. It is found as a rusty sword in an area north of the whirlpool to Crossbone Isle. Take it to Obaba to reforge it to its former glory. It could be equipped by Matthew, Tyrell, Karis, Himi, Eoleo, and Amiti, though it would be most fitting with Karis due to its Jupiter Unleash. However, Dark Dawn and The Lost Age both feature stronger weapons than those found in the original Golden Sun, so the Kikuichimonji may be overpowered by the Verdant Sword or Lachesis' Rule.

Cultural References
"Kiku-ichimonji" is the name given to the Japanese swords made by the thirteen swordsmiths who were in attendance to the Japanese Emperor Go-Toba in 1208. Asuras, in Hinduism and Buddhism, were demonic, power-seeking deities.