Unlike many of those known by a title, we know the name of The Engineer: Roger M. Ire, the First Founder of the Sky-Blue Republic and the founder of the Esoteric Engineers.
Roger was one of the minority of individuals that not only survived the Coming of the Azure Flames, but would emerge as a power into himself- like the Archmage or Necromancer did. However, he did so only after a long period sealed away from the Necromancer's eye (and thus his power).
Roger was one of a cadre of dabbling occultists that also understood network technology and computer science. When the Azure Flames hit, Roger was with several of his peers in a laboratory watching an experimental laser perform a test. The result was that all of them got swept out of the world and into a parallel dimension, but they did not realize at first that this dimension was actually the computer system itself.
Instead of being concerned about the massive undead hordes coming to consume them, they had to worry about keeping the system running while taking control of it and them using that position of power to spread their dominion until the real world could be reached once more. When that came to pass, Roger had become the First Founder and the Inner City of today--a city-state existing entirely inside a virtual world--and their blending of Old World science and even older magic had completed itself into the new science of Technomancy.
Roger's identity as The Engineer arose when the Inner City made contact with outsiders such as The Emperor and The Archmage. Knowing the power of names, he assumed this one to avoid ensnarement by others, and few realized that he had a more ordinary identity until well past the fact. Being able to bridge the cultural gap between The Emperor and The Archmage, The Engineer proved vital in coordinating the efforts of the three great powers against The Necromancer; being able to speak the jargon of his peers, and freely translate between the two, allowed him to quickly and efficiently tell the others what they need to know when they needed to know it and thus double-team The Necromancer.
In the final push, The Engineer--using his secret location beneath The Necromancer's seat of power--he direct attacked The Necromancer by shattering his locus of power, and thus his hold on his capital, Necropolis Prime. Once The Engineer's forces took control of these nodes, he deployed Omni-Conversion Engines that scrub the power of those nodes clean and turned them against The Necromancer. Cut off from the vast power he'd long ago took for granted, The Necromancer did not have sufficient means to recover from that shock attack; weakened, he fell in personal combat with The Emperor and--after centuries of terror--died himself.
Others cooperating in this final war were off-worlders intervening because they saw by oracular powers that The Necromancer threatened to spread beyond Earth and came to stop him. These surviving allies joined The Engineer, taking up the ruined Necropolis Prime and settled it for themselves once the Omni-Conversion Engines finished cleansing them and rebuilding the ruins into The Outer City with its crystal spires that gleaming in the sun. The Inner City remained secret, enforced upon those allies by binding oaths powered by Technomantic means.
The Engineer persists, but remains aloof these days. Only the leaders of the Inner City and the elder masters of the Esoteric Engineers see the true Engineer now; far more common are his simulacra, differentiated by number with the true one denoted as "Prime". Rivals and enemies often disappear if they show themselves a viable threat, only for some new henchmen to be debuted soon thereafter that bears a similar seeming under a new name.
The Inner City today is a great hub of intergalactic importance, and its true location remains a secret; everyone knows that it is a virtual world dimension, but none able to speak of it know where in the external world that computer exists- and all that try to do so, compelled or not, die horribly no matter the distance in time or space from The Inner City.
Where or when The Engineer is now, few know and fewer want to know.