The Battle of the Mirror Dimension was an open conflict that pitted Doctor Strange, Karl Mordo and the Ancient One against Kaecilius and his Zealots inside the Mirror Dimension.
After defending the New York Sanctum from Kaecilius and fighting Lucian in the Astral Dimension, Stephen Strange was selected by the Ancient One to replace Daniel Drumm, who was killed during the battle, as the Sanctum's protector.
However, Strange refused citing that he took an oath to save lives and that his involvement in Lucian's death went against everything he swore to do as a doctor. When the Ancient One claimed he was still set on feeding his inflated ego, Strange shot back by alluding to her drawing power from the Dark Dimension, which he learned from Kaecilius.
While Mordo outright denied Strange's accusation, the Ancient One remained silent and warned Strange to choose his next words carefully before leaving. The two men started to argue only to hear Kaecilius and the remaining Zealots entering the Sanctum.