The Age of Dark, otherwise known as the Age of Humanity, is an era in the lore of Dark Souls.
The Age of Dark is said to begin when the First Flame fades, thus ending the Age of Fire; the Age of the Gods. According to Darkstalker Kaathe, the Chosen Undead is destined to bring about the Age of Dark, as they are the descendant of the Furtive Pygmy.
In Dark Souls II: Scholar of the First Sin, it is revealed by Aldia that even if the First Flame fades entirely, a fragment will always remain waiting to be reignited by an Undead, creating a cycle in which the Age of Fire leads into the Age of Dark, only to be pulled back from the brink at the very last moment.
In Dark Souls III, the Age of Fire is on the brink of ending, as even after the Ashen Chosen gathers all the embers of the five Lords of Cinder, the fire does not ignite as before. Yuria of Londor urges the Ashen One to take the Flame for himself, to usher in the Age of Dark.