Yivrian is historical conlang, designed with a proto-language and a set of sound changes that derive it, and with parent and sister languages. But I made it and its family backwards. Yivrian itself was conceived first (and it was not originally designed as a historical conlang), and only after the language was originally designed did I begin to speculate on what its parent language was like, and begin to design its sisters. This is not how your supposed to do these things, but it worked out reasonably well.
The biggest difficulty that I encounter with this approach is that Yivrian is too similar to its parent Common Yivrian, and the other sister languages are too different. Since Yivrian came first and retains its pride of place, everything about Common Yivrian that I didn’t specifically intend to be different defaults to being the same as Yivrian, while the other languages (Praseo and Tsingrizhil) wind up with a much greater distance from the proto-lang.
Fleshing out Praseo for The Wedding of Earth and Sky forced me to confront this problem anew. It also presented a different problem: while the Yivrian-like proto-forms work fine for deriving Yivrian, when I take those forms and put them through the sound changes to create Praseo, the result is often very ugly.
For instance, for Wedding I had to consider what to call the diety that in Yivrian is named Aratelor. If we extrapolate backwards into Common Yivrian by the most direct route, we would reconstruct something like *arātelōra, which as you can see is very similar to the Yivrian form, and not very interesting. Worse, the Praseo generated from that proto-form is Arotlura which I don’t like at all.
So I did some speculating. First, the Yivrian ending -elor is commonly attached to the names of dieties, and for that reason it may be innovative or analogical. Furthermore we know that the stem from which this name is formed is arat- (which appears in several other words), so it’s reasonable to assert that the CY name is *arāti or something similar, and the Y -elor is an innovation.
The second step was a new sound change. I had long known that CY contained /*ð/, which has disappeared in all of the daughter languages but left behind traces. In Yivrian the normal reflexes were (I thought) /d/ and 0, but about this time I began to speculate that there had been a sound change of *ð => r. Yivrian has a lot of r‘s, and I find so many r‘s to be unpleasant outside of the particular phonoaesthetic context of Yivrian, so this seemed like a good chance to turn a certain number of Yivrian r sounds into something that wouldn’t be reflexed as r in the other sister languages.
Applying that to this case, I changed the proto-form to *aðāti — and this was paydirt. The Praseo reflex of *aðāti is Azatsi, and I loved the sound of that! I liked it so much that the name became canon: in Wedding the name Azatsi appears as the name of the diety in question, and that’s unlikely to change in the future.