If she have the same eyebrows as Diablo, I would say this demon lord is their child between Rem and Diablo.
I mean, from the previous action, it would look like he just make her pregnant and let the child out instantly.
..If Mana input makes this demon lord, Would that means Shera who tried to put magic on Rem also have influent on this somehow?
That means, this is a child from between 3 of them! Congrat, Shera, Rem and Diablo!