The ancient is the supreme long-lived sorcerer who is protecting the world from the evil especially from Dormammu of the Dark Dimension. Dormammu wants to take all planets into his dimension where no one is bound by the time and everything is immortal. Dormammu is mostly hunger for the planet earth but resistance by Ancient.
You can find about The Ancient one by just clicking on the link:Ancient_One
The ancient was the mentor of Doctor Strange who approached Ancient after car crash when his nerves were severely damaged, he has lost all his money on medically treatment could not get healed but fine out about a guy how miraculously recovered spinal cord injury. Strange reached at Kamar-Taj after meeting the guy.
Because of his arrogance and attitude Ancient refused to take him as a student then Ancient took him in when saw his passion and determination. Strange was fast learning and enthusiastic he was mesmerized by the magic and there training techniques there he learned, The Ancient demonstrates her power to Strange, revealing the astral plane and other dimensions such as the Mirror Dimension.
In the Avenger infinity war Doctor Strange allow the Thanos to take the in charge of time stone he might have created the mirror image before handing over the stone where all it looks like Thanos is winning the game but Doctor Strange might have taken his short.
Read More about Toney stark
Doctor Strange created the time lop when he went to dark dimension to make a bargain with Dormammu he made Dormammu to accept the bargain and take the Kaecilius and his zealots back to his dimension and never return for the earth. He has saved the world before he may save the world again.
When Ancient was dying, Strange had a word with Ancient where Strange got know that Ancient did not cure the Jonathan (the man with spinal cord injury) he channeled dimensional energy direct into his own body by using magic in return to serve something bigger. “Ancient told Doctor it is not all about you”, Doctor might give himself away to dark dimension in order to reverse the snap effect, in order to serve big purpose you needs to break the rules.
Ancient could not see the future of Doctor strange only the possibilities he told by he has huge capacity of good he will never lose his path because he has fear of losing which make him away from the greatness which leads toward the destruction. Ancient also told Strange that he needs to learn from Mordo his rigid strength and needs to learn your flexibility, its hint that Mordo join the Strange in Avenger Endgame.
Ancient might have return back in Avenger Endgame, could not see strange future but it’s Strange who will bring Ancient back to life where they all confront the Thanos and kill him.