In Doctor Strange, the main antagonist Kaecilius and his following perform a ritual to essentially contact Dormammu and channel his power to destroy the seals protecting Earth.
We discover that the Ancient One was also channeling Dormammu's powers, keeping her young and healthy, essentially immortal.
My question is, it seemed that the Ancient One performed the same sort of ritual at some point, but wasn't affected the same way. Why?
I don't believe, based on the way it was explained, that the two rituals were the same.
The Ancient One was accused of channeling the power of "the Dark Dimension" to maintain her youth. We don't know what this entailed, but it sounded similar to how any other magic works in the MCU -- you channel energy from another dimension into ours and it does things that otherwise violate the natural laws.
What Kaecilius did was directly invoke Dormammu. He was not merely trying to use the Dark Dimension for his own use, he was trying to summon Dormammu itself to come destroy the world. That's a much more serious and powerful invocation, so it makes sense that it would have a more drastic effect on it's user.
I believe the ritual Kaecilius performed with his followers could be the same one that the Ancient One did sporadically in order to keep her "youth".
However, I believe this is a matter of how deep did each one go. Honestly, the Ancient One stated herself that she hated drawing power from the dark dimension. Therefore, I am led to believe Kaecilius and his followers were much more connected to Dormammu than her.
Also, she never intended for Dormammu to destroy earth and force all humans to reunite with it, which was something Kaecilius longed for. So it must be a matter of intensity and how recently did one perform dark arts. I recall that when the Ancient One comes to defend Strange and Mordo, Dormammu's symbol appears on her forehead, which confirmed Strange's suspicions that she had indeed engaged in that sort of ritual.
