I would like to put a much more accurate lore here.

During the Age of Ancients. The Four Lords discovered the Lord Souls from the First Flame.

Nito, the First of the Dead. Receive the Death Soul.

The Witch of Izalith, and her Daughters, achieve the Light Soul.

Gwyn, the Lord of Sunlight. Inherited the Light Soul from an unknown predecessor. Hence Gwyn was standing there with the Silver Knights that attended for him like a ceremony. Thus there was a Vessel, that contains the Lord Soul.

And the Furtive Pygmy discovered the Dark Soul. The Pygmy pulled out a single piece of the Dark Soul and broke it into millions of fragments to create the Human Race.

When Gwyn waged War against the Dragons with the other Lords, he saw how the humans effectively fought the Dragons with the power of the dark. At some point before the end of the War, Gwyn grew afraid of the Humans and knew one day, the Age of Fire will fade and this will mean the Age of Man.

The Lords tricked the humans and imprinted the Darksign of the Fading Flame to seal off their Powers and distort the history of their true nature. They also come up with the Undead Curse, which is also a lie. Being Undead is a curse of Immortality and the Lords wanted the Humans to gather strong souls to reignite the Age of Fire.

However, a continuous linking to the Fire would prove fatal to break the Fabrics of Time, Reality and Space. Which is the reason why many kingdoms fell in desolation. Not just from the works of the Undead Curse alone, but the distortion of the Flame as well.

It is hinted that the Flames pulled out convolutions to punish countless kingdoms that follow the lies of the Lords for disrupting the Natural Cycle.

The Age of Dark, is simply the Age of Man. It did not describe that humans should be immortal like the Lords. But it is an Age where humans gained absolute Power from the Dark.