Waragai: Initially, he was conceived as an illusive purveyor of rare objects.
Miyazaki: Yes, a character like that is actually very easy to fit into the game, you can essentially place him anywhere and move him about wherever you want to. Mr. Waragai took care of the design but it wasn't until he added glasses, that the character started to emerge.
Waragai: I agree. There are actually a number of examples of armour with glasses attached to them, so I just used those as a base for the design. At first glance, the character may look eccentric but his armour is actually quite typical of suits worn in central Europe in the late Middle Ages: just a chainmail, a surcoat and leather boots.
Miyazaki: I was adamant that he should be carrying a lot of things. I think Waragai did an admirable job. Even in this world, where money has lost its value, he holds onto these precious objects and carries them along.
Nakamura: Talking of Domnhnall, didn't we design a weapon for him, the triple crossbow? I really liked it, it was very unique, especially when compared to the other weapons in the game.
Miyazaki: The triple crossbow, you mean the Avelyn? Unfortunately, I had to move the it to the Duke's Archives.
Otsuka: Avelyn was originally Domnhnall's weapon? I really like the name, where did that come from?
Miyazaki: Hmm, I think there was a reason behind it, but I've forgotten… at least that's what I'm going to tell you. I was fully aware when we were designing it, that the detail and complexity of the weapon wouldn't come across in the game but I specifically remember saying that we'll probably make an art book in the future so let's just make it. I'm glad we were able to realise that dream.
Waragai: Make sure you make it really big so as soon as you open the book, BAM! I don't work in that area much myself, but designing weapons looks like a lot of fun. The Avelyn would have to be my favourite, but I also love the design of the Dragonslayer Spear.
By taking a closer look at the Avelyn, the only weapon that mentions Eidas in its description, interesting design patterns can be found. These patterns are really similar to the ones found in the Crystalline set. This could connect the Crystal Knight with Eidas. It may also be possible that he went hollow and died in the gardens, where the set is found.
Eidas is also possibly related to Sen's Fortress by the pressure plate traps, which are evidently similar to how the Avelyn works.
Also, it's important to note that the crossbow mechanical design is similar to how the elevators and stairs work in the Archives, which is also theorised to feature similar design in some places.
In the Design Works Artbook, evidence suggests that the two were once, or still are, related. On the following image, Avelyn's concept art is on the left, while on the right, the Crystal Knight's weapon and shield can be seen.