So! Instead you get spoilery speculation on Kilvas, because that doesn't require any outside reading.
My speculation starts here (source is from Serenes Forest):
420: Formation of Kilvas
Because of differences within the bird tribes, the Ravens left Phoenicis. At the nearby island of Kilvas they established their own country.
425: Begnion Empire's invasion
Feeling that their dominance has started to fade, the Begnion Empire, using the Goddess's name, sent troops to begin an invasion to restore their dominance of the continent. The invasion started with new powers Daein and Kilvas.
428: Kilvas' surrender
Kilvas refused Phoenicis' reinforcements and surrendered to Begnion. Also the Daein troops, concentrated at the capital of Nevassa, continued the resistance.
478: Treaty of Sarasa, Kingdoms of Daein, Kilvas and Serenes established
The war entered a seemingly neverending state. Begnion, worried about their great losses, finally decided to find a way to stop the war. In the end, by the intervention of Goldoa, peace talks were held within the Sarasa region of Goldoa. Begnion signed a treaty of peace, with the conditions that they had to return their conquered lands to Daein and Kilvas and recognize them as official countries.
I speculate the king of Kilvas at this time, presumably Naesala's uncle or someone (even though this makes no sense in a culture of strength), signed the blood pact either at this time, or around the time of the Treaty of Sarasa in 478, which resulted in Begnion "recognizing" Daein and Kilvas as nations. We're never given a timeline in the game, but Lekain does tell this little story late in chapter three:
Calm yourself, young king, and listen. I'll tell you an interesting story. Long ago, a foolhardy king signed a blood pact with the empire to take back his lost kingdom. In exchange for his country, he vowed complete obedience. He soon broke the terms of the pact and began to disobey the empire. Presumably he didn't understand the power of the oath he'd sworn... Would you like to hear what happened after he defied the empire's will?
It was quite tragic. One man died on the first day. He was only a peasant, so no one really noticed. Two more died on the second day. Three more on the third day. I think you can see where this is going. The wave of mysterious deaths spread across the kingdom like a contagion. After thirty days, the king finally realized that this was the curse of the blood pact, the price of infidelity. He apologized to the empire and pleaded with them to lift the curse. His plea was graciously accepted on the one hundredth day. By then, the king had lost half of his subordinates, as well as his lovely wife and child. That country remained under the control of the senate for a very long time.
Lekain's description doesn't bring a laguz nation to mind-- but we don't know about any other entities who have signed blood pacts and also been in Begnion's service for a long time. It could refer to Ashnard's father, but the details of that situation (as described by Almedha and Sephiran) are different enough that these could be two different incidents. Also noteworthy are the illustrations during Lekain's retelling, which imply he's talking about the raven king.
It's possible Naesala was the king who signed this pact. In the interest of writing an adventure story in Kilvas with Naesala at its center, I decided to speculate instead that, after the plague incident, he decided his predecessor was an idiot and took matters into his own hands, along with the throne (it was conveniently there) and a blood pact he didn't know would be following him (which was also there, and rather inconvenient).
Which is more likely? I think it depends somewhat on Naesala's personal history, and even his age, actually; the bird tribes supposedly live up to a thousand years, though among ravens, Nealuchi is the oldest at somewhere around eight hundred. Naesala could have been a young leader, perhaps even leader of the movement in which the ravens broke away from Phoenicis, and then perhaps fell into Begnion's trap simply because he was inexperienced. It could be he was more idealistic in his youth, and was only soured after a few hundred years in Begnion's service.
Alternately, Naesala could have simply been in line for the throne, and via investigation of the plague, or a confession from the previous king, he found out about the pact and got rid of the guy - before or after the apology, as I don't think it matters who begged the senate for mercy - in the hope that doing so would free Kilvas. Then he found out that it doesn't work that way.
Now that I write it out like this, I like the first option a little more. It gives the friction between Naesala and Tibarn some depth. I don't think the assumption that they founded their nations is inappropriate, though it feels like a bit of a stretch.
I might be forgetting hints in the script that would clarify this. I really should play RD again - to take notes, if for no other reason.