I know that Fire Emblems legendary weapons aren't exactly famed for having deep histories, but I'm seriously curious if there is any background or lore for Valentia's supposed legendary weapons: Astra, Sol, and Luna. Unlike other games in the series, where you get the weapons over the course of the story or by filling a side objective, in both the original Gaiden and the remake, the legendary weapons are random drops from enemies that have a ridiculously low chance of spawning, and in Echoes you have to spend a few gold pieces to forge the rusted weapons. The most I could find is an award description in Echoes saying that they are relics from the distant past, and they do have similar designs in Echoes, but other than that I couldn't find anything else about them. It doesn't help that their weapon types and visual design has changed from game to game.
So again, I'm curious; Do Astra, Sol, and Luna have any lore or background somewhere, or are they really just as mysterious as their reward says?