system
Given the following post, respond as if you're commenting on Reddit:
human
[Dark Souls] Is living in an age of dark an absolute nightmare, or pretty much the same as living through an age of fire, aside from the fact that it's always dark outside?
gpt
So there are a couple of things that we need to establish here before we dip into this answer.

First, is that the world of Dark Souls goes through two phases. An age of Fire and an age of Dark. The Age of Fire is also known as the Age of the Gods, because the Gods were empowered by souls. Or at the very least fragments of a Lord Soul, which was pulled from the First Flame. The Age of Dark is also known as the Age of Man, because unlike the Gods, man is in possession of a Dark Soul, also known as Humanity. Humanity is an inherently corrupting force, and we've seen what too much of it can do to something(see: Four Kings, Artorius, Abyss Watchers...) The Abyss is the most extreme example of this, being composed of infinite humanity.

The second thing we need to establish is exactly what man is. Though this is a difficult thing to answer. There is evidence to suggest that the true form of humans in this world is that of the hollows, because that is the form of the Furtive Pygmy when he first pulled the Dark Soul from the First Flame. Humans are now permanently tainted by the Dark Soul, however, with each human being born with one, making their "natural" state seemingly normal to us. It doesn't seem like hollows are able to retain their human form with normal souls, however, requiring Dark Souls specifically to do so.

So this brings us to our final point. The Age of Dark comes when the First Flame can no longer hold back The Abyss, and it overtakes the world. This new surge of humanity through the world effectively makes humans the dominant force in the world, unlike the Gods during the Age of Fire. What this means is suspect, however, because if humanity is corrupting, then it is completely possible that soon anything and everything will become warped by the infinite humanity and become shapeless horrors. However, we visit the past in multiple occasions in the Souls series, and even traverse the abyss and meet people unaffected by it. So it is very possible that whatever corrupting power the Abyss holds only applies during an Age of Fire, and Fire is instead the malevolent agent.

Some people believe that Bloodborne takes place during an Age of Dark, citing that the Cleric Beasts and other monsters you fight bear a striking resemblance to those corrupted by humanity in the Souls series. Though really, we have no way of verifying that, unless you believe that the painting at the end of the Ringed City DLC was intended to be  Yharnam. After all, it was a painting born of blood. The blood of the Dark Soul, specifically.

tl; dr The flow of time is convoluted in Lordran.