All that's been set in stone thus far is the fact that there's an so-called evil nation, although via some brainstorming I've come up with what I feel is a suitable backstory.

Said evil nation is a bit smaller than average, being just one of five nations that are surrounded by a single mountian range. This nation's(For now, let's call it something rather bland, like Vinsenia) one claim to fame is that it holds onto a single item of near-mythic status, an enchanted blade akin to one of the many legends that are scattered around the FE universe. However, the reason that this blade stands out from any others is that it was supposed to glow with rippling power when one who was 'suited to be king' held it. 

Needless to say, a new king emerges who can make the sword glow brighter than it has in generations, after the death of the old king and the rumors that the rest of the royal family has stepped aside. He immediately sets out to annex the neighboring countries, and does so with almost ridiculous timing and precision, causing many of the greater nations to suddenly take an interest in this new threat to global balance. Thus, the PC group are hired by a village that's in the bath of Vinsenia's progress to defend them against the rising influx of bandits that flee the marching army.