PLZ COMMENT I NEED HELP

Ok so I’m in the works of planning out a novel. It takes place in a fantasy world but all the protagonists are from earth as well as the antagonist.

I’m thinking of making a second book (so it’ll be like a two book series) but the second book would be like a prequel explaining how the antagonist got to that world (which is actually quite interesting.) what do you guys think I should do? Write that second book or nah? It’s not really necessary, but it’s interesting and explains certain stuff.