Darkness is one of several Artifact currencies that can be found within the Underworld. It is used to improve Zagreus' power at the Mirror of Night.
||'...Born of Chaos, the Underworld is a domain of pure and utter darkness. Sometimes, this dark essence becomes manifest through all the raging of the dead, and all the strife that can unfold beneath the earth. And, there are those born of this realm who can absorb that essence, gaining oneness with the Underworld, growing stronger all the while. That darkness, having taken root within, connects the bearer to the Underworld, makes them inseparable. Almost one and the same. Those born of darkness must remain in darkness; this is one of the Underworld's indelible laws.'
Darkness can be received as a room clear reward, as a reward for opening Infernal Troves, from the Fated List of Minor Prophecies, or in trades with the Wretched Broker.
When Heat is too low for Bounties, every boss defeated will drop a large amount of Darkness instead.
With the Pitch-Black Darkness upgrade from the House Contractor, Darkness will also give +5 when collected from a room reward.