The Dark was a period of time after the Sylex Blast in 64 A.R. and before 450 A.R. when the Ice Age fully began.
During this era, in the aftermath of the blast, Dominaria's climate became colder and resources became scarce. Quaking before the climatic shifts rocking Dominaria, the peoples of every land fell under the control of zealots and despots. Because of the Brothers' War, the use of magic was condemned by the Church of Tal, and so mages practiced in secret. The people were factionalized and rose up against each other. It was a time of persecution and corruption when it seemed as if the end of the world was at hand.