Well, to address your second point first, the Empire is more or less the grouping together of realms with a hierarchy partially above the rulers of the realms, which can, to some extent, dictate their, and their realms', actions.
As for the other...I think you may have misunderstood the nature of the situation. This isn't Luria coming to Fissoa and saying, "Hey, why don't you and we get together and form an Empire, and call it the Lurian Empire?"
This is Luria Nova, sole current member of the Lurian Empire (which is, in fact, already making preliminary preparations to eventually split apart again, if all goes well, into multiple Lurian realms, united under the Imperial banner), asking Fissoa if it wants to join the Lurian Empire, which has existed, in one form or another, for some time now.
Ah. Well Luria Nova has never presented itself to us as an Empire, nor announced it's intention to split up again. Things might have gone differently if you had.
