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, countless chosen have prolonged the age, preventing the end. By now there are countless Lords of Cinder, those who successfully linked the First Flame before, and even more Unkindled who were unsuccessful. When the current chosen, Prince Lothric, neglected to burn, a contingency plan was activated where previous Lords of Cinder rise to lend their power to an Unkindled, but after reviving and seeing the state of their world, they neglected to fulfill their duties. Aldrich, one of the Lords of Cinder, decided not to take his throne after experiencing visions of the deep sea and its inevitable coming.
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. In the "true" ending the fire is let to fade, but the Fire Keeper reveals that one day another fire will ignite, and that the cycle of darkness and light can now continue as it was meant to. Yuria of Londor urges the Ashen One to take the Flame for themselves, to usher in the Age of Dark.
The Untended Graves is a mysterious location that is implied to showcase a world from the past in which the Age of Dark had settled.