So I was digging about, certainly not just a simple google search and found that the word Kamui relates to Ainu(native-native japanese) Gods/deities. While there are a few Kamui that could possible represent the one we see in fire emblem, we don't get to know much about him. Other than he is naive. However remembering the translation of his/her sword 'Night Blade God' this seems to be the intention of the naming.
Although I have a bit of a bizarre theory I want to tack onto this. So one of the Kamui that is listed on Wikipedia is called Nusakoro Kamui, the god of the dead and community. Now I believe in the video when we see Max(or Marx) take down that thing, it seems like a Risen to me, judging by the sound it makes (I hear a dying Risen at least). So if our prince is supposed to be Nusakoro (or something similar), perhaps he is bringing(most likely unintentionally) them to this world.
Granted this is all just conjecture and mad guessing, but it's just a possible thought.