Alondite

Alondite （エタルド Etarudo, Ettard in the Japanese versions) is the Black Knight's ultimate weapon, given to him by Sephiran. Along with its sister sword, Ragnell, Alondite was blessed by the goddess Ashera to be wielded by the first apostle, Altina, and so to vanquish Yune. Statistically, the sword itself is identical to Ragnell. The only differences between the two swords are color and the fact Ragnell can only be used by Ike, whereas in Radiant Dawn Alondite can be used by anybody with a high enough sword rank.

In Part 4 of the final chapter in Radiant Dawn, after Ike defeats the Black Knight, the Black Knight dies, and both swords start glowing, as if one cannot exist without the other. Ike will pick up Alondite, which may then be given to any character of the player's choice. Alondite immediately becomes one of the best possible weapons for swordwielders (Trueblades particularly) and can be blessed with Yune's blessing to make it more potent. If you have trained up a second powerful sword wielder, this would be the blade to give to them (the first person would have the Vague Katti.)

An interesting characteristic of Alondite is mentioned by the Black Knight in Chapter 9. After Alder blocks the Black Knight's attempt to kill Jarod, he says that the sword was not made to kill those who cannot defend themselves. This would explain the Black Knight's failure to kill Ena--even in an execution setting--in Chapter 28 of Path of Radiance. It is not known whether this is also characteristic of Ragnell but it seems a fair guess.

It is safe to assume that Alondite, and possibly Ragnell, were returned to the Mainal Cathedral after the battle with Ashera.

Fire Emblem: Path of Radiance
|Alondite |Sword |Prf* | - |18 |80% |5% |1-2 |20 |? | - |Negates non-Wrath criticals, Defense +5

* Preference (Black Knight)

Fire Emblem: Radiant Dawn
|Alondite |Sword |SS | - |18 |80% |5% |1-2 |20 |2 | - |Def +5

Etymology
In some Arthurian texts, 'Alondite' is named as Lancelot's sword.

Trivia

 * The Black Knight's sword was originally called Ettard in the Japanese version of Fire Emblem: Path of Radiance. When it was localized for North America, they changed the name to Alondite. Then when Fire Emblem: Radiant Dawn came along, the sword that Ike joins with, which is not the Black Knight's sword, is named Ettard in the North American version, and Alondite in the Japanese version.
 * In the North American version of Super Smash Bros. Brawl for Wii, the Black Knight's trophy states that he wields Ettard. This is most likely due to the above stated reason that Alondite was called Ettard in the Japanese version of Path of Radiance and Radiant Dawn.