The Guardian Aegwynn grew powerful over the years and used the Tirisfal energies to greatly extend her life. Foolishly believing that she had defeated Sargeras for good, she continued to safeguard the world from the demon king's minions for nearly nine hundred years. However, the Council of Tirisfal finally decreed that her stewardship had come to an end.
The Council ordered Aegwynn to return to Dalaran so that they could choose a new successor for the Guardian power. Yet Aegwynn, ever distrustful of the Council, decided to choose a successor on her own.
The proud Aegwynn planned to give birth to a son whom she would divest her power to. She had no intention of allowing the Order of Tirisfal to manipulate her successor as they had tried to manipulate her. Traveling to the southern nation of Azeroth, Aegwynn found the perfect man to father her son: a skilled human magician known as Nielas Aran.
Aran was the court conjuror and advisor for Azeroth's king. Aegwynn seduced the magician and conceived a son by him. Nielas' natural affinity for magic would run deep within the unborn child and define the tragic steps the child would later take. The power of Tirisfal was also implanted in the child, yet it wasn't to awaken until he reached physical maturity.
Time passed, and Aegwynn gave birth to her son in a secluded grove. Naming the boy Medivh, which means "keeper of secrets" in the high elven tongue, Aegwynn believed that the boy would mature to become the next Guardian. Unfortunately the malignant spirit of Sargeras, which had been hiding inside her, had possessed the defenseless child while it was still in her womb. Aegwynn had no idea that the world's newest Guardian was already possessed by its greatest nemesis.
Certain that her baby was healthy and sound, Aegwynn delivered young Medivh to the court of Azeroth and left him there to be raised by his mortal father and his people. She then wandered into the wilderness and prepared to pass into whatever afterlife awaited her. Medivh grew to become a strong boy and had no idea of the potential power of his Tirisfalin birthright.
Sargeras bided his time until the youth's power manifested itself. By the time Medivh had reached his teenage years, he had become very popular in Azeroth for his magical prowess and often went off on adventures with his two friends: Llane, the prince of Azeroth, and Anduin Lothar, one of the last descendents of the Arathi bloodline. The three boys constantly caused mischief around the kingdom, but they were well liked by the general citizenry.
When Medivh reached the age of fourteen, the cosmic power inside him awakened and clashed with the pervasive spirit of Sargeras that lurked within his soul. Medivh fell into a catatonic state which lasted for many years.
When he awakened from his coma, he found that he had grown to adulthood, and his friends Llane and Anduin had become the regents of Azeroth. Though he wished to use his incredible newfound powers to protect the land he called home, the dark spirit of Sargeras twisted his thoughts and emotions towards an insidious end.
Sargeras reveled within the darkening heart of Medivh, for he knew that his plans for the second invasion of the world were nearing completion, and that the world's last Guardian would bring them all to fruition.