The Darkness' Lair is a location used by The Darkness as a hideout.
It is seen for the first time in Hell's Angel, where The Darkness, aka Amara is resting as she allows Rowena to heal wounds inflicted upon her vessel by the angels sometime earlier. After Rowena has completed the task, Amara tests her strength by sending a shockwave towards Heaven in return.
Finally at full power, Amara thanks Rowena for her work and orders her to spy on the Winchesters to see what they're up to. Rowena lies that they have no plans yet, and when Amara isn't looking or has gone somewhere, Rowena flees to switch sides and help the Winchesters.
The Winchesters plan to trap Lucifer and expel him from Castiel's vessel. The plan fails however, and Lucifer proceeds to suffocate the Winchesters. Amara returns to the lair at some point during this, and noticing Rowena is missing, tracks her down to where Sam, Dean and Lucifer are.
Lucifer attacks Amara using a Hand of God, but fails to do any damage. Amara then kidnaps Lucifer and brings him to her lair in hopes of using him to lure God out. Lucifer suggests they work together, and that God may not appear anyway, but Amara decides to test this theory and begins torturing Lucifer over a period of weeks.
The lair is seen again in All in the Family. This time Amara has Lucifer magically-bound to a contraption and after getting annoyed with his witty comebacks, she knocks him out. She then sends Dean a vision of Lucifer's current state to get him to bring God to her, since she knows he has surfaced.
Dean instead formulates a plan with help of Sam, Metatron and Donatello to rescue Lucifer so he can help them defeat Amara. Dean lures Amara to a forest and acts as a diversion. Using Donatello's ability to locate Lucifer, Sam is able to drive to the lair.
Metatron proceeds to undo the bindings while Sam gets Lucifer agree to work with them. Metatron successfully frees Lucifer moments before Amara realizes that Dean is hiding something. Donatello warns the gang of her arrival, and three of them rush to the Impala, while Metatron voluntarily stays behind.
Metatron draws a unique banishing sigil and when Amara appears, he activates it. The blast has no effect however, and Amara questions if he was joking by using it. Metatron pleads with Amara to "spare the Universe", causing her to get angry, and she implodes him in response. She then sets out to kill the trio who left, only for God to rescue them.
On the outside, the lair appears to be an abandoned silo located in a construction area. Through the entrance there's a downward platform that leads to a wide, open interior with cylinder pillars. Light shines through the entrance, while the inside is lit by various torches, making it fairly dark.
Lucifer was bound on a high platform in the center of the room during his time imprisoned here. While Rowena healed Amara, Amara rested on a table near one side of the room.