Battle of the Mirror Dimension

"They can't affect the real world, but they can still kill us. This wasn't cleverness. It was suicide."

- Karl Mordo to Doctor Strange

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.

Background
"She is not who you think she is."

- Doctor Strange to Karl Mordo

Following the Battle at the Sanctum Sanctorum, the Ancient One confronted Doctor Strange and Karl Mordo, proposing to Strange that he be the master of the Sanctum Sanctorum, having considered Daniel Drumm's death. However, Strange argued with her about drawing power from Dormammu to prolong her life, leaving Mordo disillusioned.

Battle
"The Mirror Dimension. You can't affect the real world in here. Who's laughing now, asshole?" "I am."

- Doctor Strange and Kaecilius

When the Ancient One left Doctor Strange and Karl Mordo, the Zealots attacked. The fight led to a chase in the streets of New York City. With the battle spiraling out of control, Doctor Strange sent everyone into the Mirror Dimension in an attempt to contain the situation. Kaecilius revealed, however, that his connection to Dormammu granted him more power inside the mirror reality. Strange and Mordo promptly stole Kaecilius' Sling Ring from him and fled.

As Strange attempted to use his Sling Ring so he and Karl Mordo could escape the dimension hoping to leave the Zealots stuck without a Sling Ring of their own, Kaecilius distorted the terrain around them and made Strange lose his focus. The Zealots gave chase and warped nearby buildings and skyscrapers, causing Strange and Mordo to plummet down the New York City landscape, but landing unaffected. Kaecilius and the Zealots eventually manage to separate the two before cornering Strange.

Suddenly, the Ancient One reappeared and rescued Strange by warping the dimension herself, confirming the fact that she was drawing power from the Dark Dimension after the dimension's symbol appeared on her forehead. She engaged in combat with Kaecilius, but he quickly gained the upper hand, however, and stabbed her before pushing his former mentor out of the Mirror Dimension and back into the Earth Plane, where she fell several stories to her death.

Aftermath
The Ancient One was taken to Metro-General Hospital, where Christine Palmer, Doctor Strange and Nicodemus West attempted to save her in vain. Before she died, The Ancient One and Strange had one last talk, which motivated Strange to fight Kaecilius and helped him understand why she was borrowing power from Dormammu. Mordo on the other hand felt betrayed and infuriated that the Ancient One was breaking her own rules.

The Zealots traveled to the Hong Kong Sanctum and destroyed it, prompting Strange and Mordo to follow.