In order to reverse the Decimation, the surviving Avengers went back in time to retrieve the Infinity Stones. Bruce Banner, Steve Rogers, Tony Stark and Scott Lang traveled back in time to the Battle of New York in 2012 to retrieve the Space, Mind and Time Stones. Banner sought the Time Stone and met the 2012 Ancient One.
I know that she is steadfast, but unpredictable; merciless, yet kind.
~ Karl Mordo
According to Karl Mordo, the Ancient One is "steadfast, but unpredictable; merciless, yet kind." This is shown when she was willing to kill multiple Zealots in combat and initially refuse to help Stephen Strange when he first came to Kamar-Taj due to his arrogance, though she did so for the sake of protecting the world from the rogue sorcerers and to ensure that other sorcerers did not become like Kaecilius and his followers.
While she enforced rules on the Masters of the Mystic Arts against breaking the laws of nature or using their power for god-like feats, such as manipulating time or becoming immune of death, she was willing to break her own rules to continue to personally ensure that they were followed and that the world was safe, though she never wanted to do so.
This lead her to draw power from the Dark Dimension to become immortal, an act which she herself forbade. Since Doctor Strange was similar to her in her breaking the laws of nature to save the world, she believed that he and Mordo, the latter of which was completely devoted to following her rules, would need to balance each other out in order for them to protect the world when she died.
Powers & Abilities
Master Sorcereress: As the Sorcerer Supreme and the leader of the Masters of the Mystic Arts, the Ancient One was the most powerful member of the order, possessing tremendously mastery and knowledge of the mystic arts, which she uses to defend his native reality. Having successfully prevented countless threats during her long life, the Ancient One's immense power and mastery of the mystic arts was very well known throughout the Multiverse, such that her presence on Earth was sufficient to keep many threats from the other dimensions from invading.
Eldritch Magic Manipulation: Ancient One was able to create, shape and manipulate Eldritch Magic into weapons, shields or spells.
Teleportation: Using a Sling Ring, the Ancient One was able to open a fiery portal to another location, gaining ability to move tremendous distances instantaneously.
Interdimensional Travel: Ancient One was capable of traveling from the Mirror Dimension to Earth. Additionally, she was able to project Strange's consciousness across various dimension's of the Multiverse.
Astral Projection: Ancient One was capable of separating her astral form from her body and accessing the Astral Dimension. She notably pushed out Stephen Strange's astral form from his body and later communed with him in the Astral Plane in her final moments. When an alternate version of her interacted with Bruce Banner, she was able to project his astral form from his body and interact freely with him without separating from her physical body.
Chronokinesis: Using the Eye of Agamotto, the Ancient One was able to control the flow of time, be it on a small or massive scale, she was known to be able to use the Eye to gain a limited power to peer through time. Within the Astral Plane, she was able to slow time around her to a crawl so she could watch the snow fall before her death.
Telekinesis: The Ancient One was able to influence objects and people with mere gestures. In the 2012 Time Heist, the alternate Ancient One moved the body of Hulk towards Bruce Banner's astral form to return it to his body. While using this ability, a golden mist appeared around the object she was influencing.
Immortality: Ancient One draws power from the Dark Dimension to fuel her ageless life. However, she can still be mortally wounded.
Reality Warping: Ancient One's connection to the Dark Dimension allowed her to warp space in the Mirror Dimension. It was unknown if she could achieve this feat on Earth like Kaecilius could, but it was likely that she can due to her being more powerful than him. Ancient One also displayed being able to counter a shockwave from Kaecilius's warping of reality and even repel Kaecilius also by slamming her hand to the ground and summoning a shockwave of her own.
The Ancient One's death.
The Ancient One in Avengers: Endgame.
Unlike previous incarnations, the Marvel Cinematic Universe's version of the Ancient One is female instead of male.