Just a Theory
The Voln which is not known by it's current name is an ancient clan dating back to the Zhcted civil wars (Hence having the same pantheon of gods), it's the strongest clan, even stronger than the 7 clans that vowed loyalty to the Black Dragon King. It decided to be independent hence, they formed the 1st clan that lived in Lutelia. And what about the Black Bow and the Durandal? The Black Bow and the Durandal were the key that allowed the clan to hold their ground against Zhcted since it's so powerful than any draconic tool (which can only do major destruction on the battlefield but not on nuking cities(at the cost of his life). Soon however the old clan begun dying out because of it's isolationist policies, hence it's descendants became a minority in Brune only known as the Voln and several minor families, the Voln took their heirloom with them: The Black Bow, as for the Durandal, it's forgotten because none of the old families can wield it. Charles then finds the sword in question and through a prophesy (I don't know if Tir na Fal has something to do with it), Becomes the King of Brune through the sheer power of the sword. It is then passed on, while the Voln family, knowing that the Bow is a great Hazard if it's wielded by a person seeking only power through cruel means(Thenardier and Ganelon are prime example) decided to keep a low profile by lording over a frontier territory in which no one would like to stay in: Alsace. This belief has been passed through the Voln family (along with the bow, though they never disclosed about it to their descendants) and it's the reason why Tigre still prefers staying in Alsace and living a quiet life (You can blame all his forefathers for that one).
Another one for the bow:
A hunch from the mural, Tir na Fal controlled the dragons using the Black Bow, and Brune destroyed all statues of the Bow Goddesses, well since Tir Na Fal is considered a taboo, that is to be expected, and that the reason why they have so much hate about it.
The bow has conditions for it to activate:
1.) One must make a blood pact with Tir Na Fal.
A blood contract that involves having one's blood spill on the bow, as seen on volume 1, The bow only reacted to Tigre when his palm was wounded.
2.)The bow is powered by thoughts.
Two cases here, either positive or negative.
When he's with a Vanadis, positive thoughts such as Trust allows him to enchant his arrows with the element of the Vralt he is close to.
When he's alone, negative thoughts such as hate, allows him to fire arrows imbued with the darkness attribute.
Well feel free to debate it here...