As I understand this:
Secre tried to seal Devil, Devil countered with word magic and reincarnation magic hoping he will be born in era where he could get rid of these two meddlesome people ( prince and Secre). Then to counter that reincarnation magic when time comes she sealed the prince and herself.
Prince/First Magic Emp died along with Licht. He was titled posthumously, I suppose making him the first emp. Royals just left his sealed body on top of the demon and started the post in his honour.
Devil reaction when he sees the Lumiere ! Poor chap got revived barely 15 minutes ago, after waiting for five centuries, and the same party poopers show up again.
There are so many questions still... If using the stones can turn you into something else, is it another entity, Licht summoned or his own soul partly turned into the demon king and that half of his soul was bound to his grimoire?