In grand video game RPG fashion, there isn't really a good or evil here. They are grand primordial energies, with subtle motivations that are distinctly inhuman. While they do encourage recognizable traits when filtered through a mortal soul, neither has a monopoly on right and wrong - both influence those attuned to them in positive and negative ways. There are numerous religions and philosophies that try to interpret (or, more cynically, fabricate) the desires of these forces for various reasons, and just as many individuals who treat them as one more source of power.
Which is all well and good I suppose, but what does that mean for the average Player Character?
Most of them will begin play free of Allegiance; which is to say, they have 0 points in both Light and Darkness. Various happenings will increase your score in either of these two pools, including but not limited to;
- Opening a Path of Magic
- Taking an Elective Ability marked as a "Boon"
- Travelling unprotected in corrupted/blighted areas for an extended period of time
- Making a Pact with a Deva, Asura or Dragon, or completing a task for them
- Being brought back from the dead (assuming it doesn't go horribly wrong)
If the amount of points accumulated in either Light or Darkness ever exceeds the other by two, you are considered to be aligned with that side. If you have points in both, but neither is greater then the other by the above margin, you're considered to have a Twilight Allegiance. Gaining a high amount of points in either will also result in "Gifts": changes to oneself that generally have positive and negative effects that are proportionate to one another. People who've devoted themselves to these forces are always slightly odd, at least as far as normal people are concerned.
Beyond Gifts, these scores have an effect on how certain things react to you. Magic Items have Allegiance point requirements (and limits) for their use, spells and other effects wax and wane in potency when used on aligned individuals and otherworldly creatures might actually notice you. Some devices even depend on these scores; there is at least one door that will not open for someone with Darkness in their heart.
Many characters also have the choice of ducking out entirely - they can avoid magic and boons and remain unobligated in cosmic power struggles beyond the ken of ordinary mortals. But ask yourself this; Did anyone ever manage to get sweet glowing eyes by fence-sitting? I don't think so.