To tell the story of Medivh we must start with the origins of The Guardian, and to tell the story of the Guardian we must talk about where mages come from in the first place. The Well of Eternity, the lifeblood of the Titan we know as Azeroth, is the origin of all magical knowledge. The trolls that lived in proximity to it were transformed slowly, over time, by its energies and became the elves. These elves were more intelligent and curious about the well, and what made them different, and over time learned to use the magic that had changed their very being. They used their new knowledge to defeat the trolls, their ancestors, and build the great kingdom of the Kaldorei.
After the Sundering, when the Night Elves rejected magic and exiled any who still wished to practice it, the High Elves travelled far away to create their own kingdom where they could practice magic to their heart's content. Eventually they came under attack by the trolls of Zul’aman who lived in that area, and decided to share their knowledge with only 100 humans of the kingdom of Arathor, in exchange for military aid. It is from here that the knowledge of magic spread to the gnomes and eventually the dwarves as well. As time passed, humans, with their shorter lifespans, forgot the rigorous discipline of the elves and began to experiment more recklessly with magic, accidentally tearing the fabric of reality and allowing demons to push their way through. At this time many of the similar thinking mages had been drawn together to the town of Dalaran, and they tried to keep their accidents from the knowledge of the regular citizens, not wanting to cause panic. So they went to the elves. The elves, being long of memory and life, told them the tales of the Sundering and the Burning Legion, and warned them of the dangers of abusing magic. Together they decided to form the Council of Tirisfal, named for the secret location where they would meet, and pour their magic into one person that they chose, in order to make them strong enough to fight the occasional demon that made it through, and also to gather knowledge and power should another attack ever come. The first Guardian thus imbued with the council's power was called Alodi. Using the Forge of the Guardian, which he helped to create, the members of the council poured their power into him, so much that he was able to single handedly fight the demons of the Burning Legion that managed to make their way into Azeroth.
Aegwynn was Medivh’s mother, the most powerful of all the Guardians, and the next part of our story. As an apprentice, Aegwynn proved the extent of her powers by reading and mastering the Meitre scrolls, a feat that was not usually even attempted until mages had completed their apprenticeship. She vastly outperformed her male peers, to their dismay, and was therefore chosen to be the next Guardian, the only female ever to achieve the honor.
Upon becoming Guardian, Aegwynn's first task was to vanquish the Demon Zmoldor and here we learn something about her personality. The demon had been possessing children in a nearby schoolhouse and, discovering his location, Aegwynn rushed in and vanquished him immediately before the children could be harmed. The Council was upset because they had been unable to learn anything about the demon and his ultimate plans, and they told her she could have endangered the whole world by acting too quickly. But Aegwynn didn’t care, the lives of the children were more important. Earlier Guardians had become little more than puppets for the council, enacting their will in forming the world’s future as they saw it, but Aegwynn would not be manipulated or controlled.
“Something had always perplexed Aegwynn about Meitre, the author of the scrolls. While studying his writings, she realized that the elf had wielded immense power-more than any regular sorcerer should have been capable of. After Aegwynn inherited the mantle of Guardian, she became obsessed with finding out how. Aegwynn discovered another series of lost scrolls written by Meitre. They described an arcane being named Aluneth in detail, and even included spells the elf had used to tap into the being's power. Aegwynn believed she could use Aluneth as a formidable weapon against the Burning Legion. Yet unlike Meitre, she would not simply tap into the entity's energies; she would bring the being into Azeroth and bind it to her will. Aegwynn summoned Aluneth with ease, but it would not obey her commands. The fickle creature thrashed against the Guardian's containment magics and nullified her spellwork. Aegwynn reveled in the challenge of taming Aluneth. After many setbacks, Aegwynn finally bound the entity to an enchanted greatstaff. The task of containing Aluneth was done, but it would take the Guardian years to truly harness its power.”-WoWpedia
As her power grew Aegwynn began to sense demons far to the north and she travelled to the icy continent of Northrend to hunt them down. It is here that she encountered Sargeras, or at least his avatar. Her power was so great that she was able to defeat him with ease, but as she raised Aluneth to conjure up an arcane storm that would destroy him utterly, Aluneth refused to respond. Setting the staff aside Aegwynn raised her more reliable weapon, Atiesh, greatstaff of the Guardian, the weapon passed down over the years from Guardian to Guardian, and renewed her attack as Sargeras had taken this moment of weakness as an opportunity to muster his powers against her.
When she finally defeated Sargeras, Aegwynn locked his mortal shell away at the bottom of the sea in a place that would be known as The Tomb of Sargeras, so that he could never be revived.
After this, she got a bit cocky, after all, she had done what even the council could not, she had defeated the Lord of the Burning Legion. She didn’t know that this had all been part of Sargeras’ plan. At the time of his death, when she was weakest, his spirit entered her body, and would live in the darkest recesses of her mind for many years. Waiting.
Aegwynn already had her doubts about the machinations of the council, after her fight with Sargeras she became even more withdrawn and they didn’t have contact with her for many years. Her fears amplified by Sargeras in her head, she decided not to pass on the mantle of Gaurdian, thinking that the council would only choose a puppet that they could control. So she decided she would choose the next Guardian herself, and pass on her power to them. At this point the council finally had enough of her disobedience and sent magi hunters after her. Locking the great tower of Karazhan that she had built, so that no one would be able to abuse its knowledge, she fled and hid. Eventually one of the hunters managed to catch her, but he discovered that she was not the traitorous rebel that she had been made out to be, and she discovered that he didn’t trust the council either. Nielas Aran and Aegwynn fell in love and decided they would pass the power of the Guardian on to their child. When Medivh was born, Aegwynn locked her power away inside him, along with knowledge of the council, to manifest upon his maturity, then she left him with his father and went into hiding, watching her beloved Medivh from afar.
And this is where Medivh’s story starts.