The Shadowlands are, as their name implies, dark. They don’t have a sun.
Maldraxxus’ sky is sometimes dark, sometimes even darker with ghosts.
Revendreth is also very dark, except in the places where the Holy Light of Creation is burning them to a crisp. Those places are bright, but they are also anomalous and we know exactly why. Naughty Denathrius!
Ardenweald is also very dark. The whole theme of the zone is “forest at night”. And also “Disney fairy nonsense”, but that’s unrelated.
So the Shadowlands are super dark… except Bastion. Bastion is super bright. So much so that when non-Bastion NPCs go to Bastion they usually complain about how bright Bastion is.
So the question is: Where does that light come from?
A giant soul lantern. Those individuals who cannot give up their memories and do not join the forsaken are sent there. Once there, they burn for eternity to provide light.
Very dark =/= completely dark. There’s still something lighting them up, or we couldn’t see anything unless we were next to a visible source of light. It seems like the very “atmosphere” of the Shadowlands has an innate glow of some kind to it. Maybe it’s all the anima? Maybe it’s just magic? Maybe it’s Maybelline?
Whatever it is, it’s just cranked up to 11 in Bastion, because reasons.
Have you ever been in a cave in real life? You can’t see a thing it’s so dark. It’s kinda hard to play a game if you can’t see anything. They use glo sticks.
ooooh… did’nt see that thing before… then again, i’m avoiding any info about upcoming content, so… makes sense i did’nt see it. but again, who knows? maybe it’s a big lightbulb made from anima. i mean, it would make sense, most of their tech is made with anima… maybe it’s a big blue ball of anima, but with a filter on it.
oh! maybe it’s all the anima bastion needs, but it’s so far away they can’t even fly-out to get it!
Where does Bastion get its light? Well, the realm is the shining center of the Shadowlands! The Kyrians’ pure, radiant souls manifest in a bright, beautiful glow that immerses the entire realm in glorious light! That’s why Bastion is a beacon of hope and beauty to the lesser covenants!
Serious (boring) answer: planes of existence don’t follow the standard rules of reality. This has been true in WoW, DnD, TES, and so on.