To the OP, A big piece of it is just assumed knowledge. The whole Eltarel thing is literally the last module WotC put out for PnP DnD. Eltarel was dragged in to hell and the adventuring party starts in Baldur's Gate and eventually travels to hell to save the city and its people, all while figuring out how and why it was taken to Hell in the first place.
Now, I'm not saying this is a solution since I hate the laziness of dev's to rely on this approach, but you can always look up this info online and pretty easily find it. I would prefer an elegant ingame solution to this but atleast you can have something.
