Three Planets in Time

Humanity lives on a planet unlike Earth, with horned people walking among them. These sufficiently advanced beings have traveled from different timelines from separate futures, back to the past of their shared homeworld. Why are they here? What mistakes have they come to prevent? Where is this place, and how does humanity live?   The design goal here is to create a world of angels, demons, tieflings, and cambions with all of the moody atmosphere and none of the religious references. The simplest way of sidestepping real world mythology is by moving toward sufficiently advanced sci-fi instead, while maintaining a fantasy or modern skin. Let's make some fictional planets for the bright and dark factions (whatever I end up calling them). Set the main action on a human planet so we can have a familiar backdrop and reasons for why hybrids exist, as well as an easier time running a LARP should we choose (Earth is a human planet, after all).   I hope you enjoy my experiment.