So anyway, as we know, none of these three worlds connect so timelines are different
Let's start of with a fourth world. Where a powerful country has taken over, well....EVERYTHING. all who resist are eliminated without a pause. The Emperor still wants more, so he puts his finest mages together to find a way to another worlds(You seem to get the idea where this goes to), The commander of the Secret Intelligence Corp, who has learned the ways of the ancient magic, finds a way to do that. And the quickly discover the events that had occurred in these three worlds.
Being most interested about the Demon King and quintessence (In FE7 the main villain, Nergal, learned to harness the power of quintessence, and use said power to create life, morphs, or emotionless human clones born of heresy.Who appear to be human but are actually recycled quintessence (life energy/force). Serve as the minions of Nergal. Morphs are characterized by dark or pitch-black hair, golden eyes, pale skin, and unusually red lips. Some are incredibly weak, some are unimaginably powerful, depending on whose quintessence they are made from, and the purpose of the morph.
Anyway, they said their researchers to find out how to harness the power of quintessence. And research about the Demon King and his minions, the ancient monsters(you know, zombies, centaurs, gargoyles etc.)
Using these to create a MASSIVE army of the undead, they invade Magvel, simply overpowering the people with great numbers. Why Magvel? Because of the Demon King(duh) the power hungry rulers pla is to reincarnate the demon with quintessence. However, such being would require LOTS of quintessence, they had need to invade another land.