The Archmage's name is lost at this time. It is not considered important to find it as there is no pressing matter that requires such knowledge. Instead, it is enough that we know how The Archmage became known as such.
The Archmage, in the waning years of the Old World, led a double life. To all but a few he was an ordinary, if conservative, man from a faded family formerly of means. During his life before the Azure Flames, he restored the family wealth. Unknown to all but that same few, he employed sorcery as part of his greater array of knowledge and used that mastery of magic covertly to ensure that parts of his business plans that otherwise were left to chance would not be so vulnerable.
In time The Archmage gathered around him a handful of proteges, many his own kin and the rest married into his family. They ran the restored, and growing, business while he did the key work of securing future opportunities for further growth and development. What none but his chief lieutenant perceived, until the very end, was that The Archmage had a clear vision of the Coming of the Azure Flames and used all of this time before to clear away a space and secure it for the benefit of himself and his people.
This space centered around the structure we know now as "The White Tower", a tall and strong artifact of the Old World's latter-day architecture built in a now-vanished city on the northern end of the west coast of the Old World's last great imperial power. We mock them as "The Wizards of the Coast" at our peril, for even now their power can reach across centuries of time--and, increasingly, light-years of space--to afflict us for the trespass of disrespecting their knowledge and mastery of powers that only now are as widely known as the secrets for faster-than-light travel.
When the Azure Flames arrived, The Archmage and his people remained secure in the space he and his proteges prepared previously. The people watched out shielded windows as the blue flames ruined the city about them, consumed the flesh of the people therein and cast down the Old World that once was. The Archmage and his men, on the other hand, conducted their most ambitious rite during those three days and did what was formerly impossible: they transformed themselves into a new form of Man entirely, escaping the flesh of old and becoming something else--but still Mannish--entirely.
This, reader, is how The Archmage escaped the power of The Necromancer- and, in time, so did his people. It is also how The Archmage became one of the powers that, at the end of the Wars of the Damned, would unite to cast down and destroy The Necromancer forever.