I've been planing out a sequel series to my trilogy, which has a secret society called Nukui-Paub spearheaded by the sequel trilogy's primary antagonist Ma'dtuth, who draws inspiration from Vandal Savage, Apocalypse, and Dragon Ball GT's Baby. Ma'dtuth is the former ruler of an offshoot of humanity called the Bassirid (which he aims to resurrect via time travel shenanigans) as they once ruled the hyper-advanced nation of Dosham (what we now call Atlantis) with descendants that influenced the development of all Indo-European and Afro-Asiatic cultures. It's suggested that Ma'dtuth used Nukui-Paub as a means to shape the history of various ancient Near Eastern civilisations in the name of building a utopia embodying his Darwinian principles.
But, the problem is neither Ma'dtuth nor Nukui-Paub even get a passing allusion or mention in my original series. I fear that potential readers (if I'm lucky to have any) will find the idea of both entities spending God knows how long lurking in the shadows rather than playing a significant role in the last trilogy hard to swallow. Or worse, think that I pulled both ideas out of the deepest reaches of my colon.
The worst example I can think of is Halo's Forerunner Saga novels (a prequel series set millennia before the events of the games) reveals that millions of years ago, that humans once ruled a galaxy-spanning empire that rivalled the Forerunners in power… and nor do 343 Guilty Spark, Cortana or any Bungie-era background materials mention this at any point (considering the previous two's vast knowledge on everything Forerunner-related). Even worse is that after the Forerunners devolved mankind back into the Stone Age, they could create vehicles such as airships and steam-powered ships, which would have been beyond impossible for Pleistocene humans to develop. And pre-343 media never even depicts prehistoric humans as having the capacity to create such things.
So, when all is said and done, how could I avoid making such a revelation come off as half-baked?