[You may want to read the previous post: Mystara’s Enemy Number One]
From my point of view, beyond its vastness, the Mystara setting relies on some big pillars, and makes it a unique setting compared to all the TSR/WotC products.
What I mean is that it is possible to leave out the turtles or even the whole Savage Coast and the setting would still survive. On the other hand you cannot absolutely leave out the Radiance with all the relations by which it has forged the setting’s identity, in the past and in the future.
Trying to give depth and coherence to my campaign, that revolves around the Wrath of the Immortals, I went deep on all the Radiance related themes, especially its alien origin.
Of course in the main adventure Fury of the Immortals, you may find several references on how the Radiance got to the planet and, through several changes, how it has become what it is now. But for me the main sources are, without a speckle of a doubt, four adventure modules written by Dave Arneson connected to his Blackmoor  setting (most of all DA2 – Temple of the Frog and DA3 – City of the Gods).
I must confess that these were adventures hidden in my chest for many many years and that I bought mostly for collection purposes beyond a real intention to play them.
But one day, almost by chance, I found an intriguing image on the internet, that completely turned around any prejudice I could have on Arnesons’ modules. It was the cover of a module that I learned, there and then, does not exists! Such a shame!
Better, it would have been a whole series of adventures that could/would have been the natural sequel to the DA series. 
Between all of them there was one with this strange creature, difficult to describe and quite weird, It was the Egg of Coot.
Deepening through the Blackmoor background, reading more or less all that is available (either official sources or otherwise), I could not find any satisfying answer to its real origin.
The more information I got and the more I tried to get trough in this subject to get its secrets, the more I realized that the typical “something unsaid” was left there for the Dungeon Master, to give him the ability to create his own taste of Egg of Coot.
Thinking about how Blackmoor was also the first setting created for Dungeons & Dragons® and the fact that the Egg was, in a manner of speaking, the first and greatest nemesis, it sure helps to imagine the role of the Number One Enemy for it.
So how is “my” Egg of Coot? What omelette or frittata will you make of it?
Sure it is something “BIG” and unique but before getting to an answer on this I must point out a second peculiar aspect in Mystara, essential to grasp the whole picture.
This world contains, inside itself, a whole
created by the Immortals as an experiment to preserve some races at risk of extinction.
In this cradle of races and cultures we can find everything, from Neathars to Azcan, from Nithians to Milenians.
Everyone was given a second chance, or so it seems.
If not, then they can count on a second coming (this is true for Alphatia after Wrath of the Immortals), not withstanding all the courses by which they were extinct in the Outer World.
Everyone, Blackmoor excepted. Why?
That is the question, but the official sources do not help us at all. Whether this is intended or not by the authors, I do not care at all. Blackmoor does not exists anymore and did not save itself from annihilation. And if someone asks why is it so, as a Dungeon Master I have to answer to this.
The one I got myself is that Blackmoor civilization was not saved by Immortals’ will. They got the perfect timing, the Great Rain of Fire, the self inflicted quake the blackmoorians used to get rid of the Immortals.
Maybe the DA series unwritten modules (and perhaps only in Arneson’s mind) would have filled the void between the events narrated in the first four modules (the birth of the Realm and the quake that obliterated it). And so it happens that even for those five centuries in Mystara history (between the Beagle’s arrival and the holocaust) there is not much written in the sources.
All that we can imagine is that Blackmoor civilization has managed to quickly evolve, technologically speaking, thanks to aliens and their artifacts. This evolution brought Blackmoor to develop the ability to use an energy form that you could easily compare to nuclear energy.
How this kind of energy can be coherently used in an high-fantasy context like D&D and Mystara, it’s something that we Master have to understand. To me, for example, the technology developed by Blackmoor mixes and confuses itself with the magic the planet is permeated with. (My) Intention is to have something that brings to memory a story like the one of Star Wars saga, for example.
DA2 – Temple of the Frog cover
Laser swords and gun yes, robots and androids too, even flying ships but without losing sight the magic, the true strength of the fantasy genre. A magic that from a point oozes and strengthen the technology but on the other side makes it dangerous and unstable.
The first sign of the danger and the instability of the connection between magic and technology is found on the official sources: the Beagle spaceship coming down thanks to the magical field surrounding the planet, the Great Rain of Fire, and then the Blackmoor relics that even centuries after sparkle quakes and various dangers.
Techno-magic developed in Blackmoor has to be something pulsing, nearly alive, unique, like a common source where one is everything and everything is one.
Something that, if anything happens in the city of Blackmoor, instantly spreads in the rest of the planet, with no chance to escape its effects.
Who gains from this techno-magic?
Well, in the beginning, perhaps, the Blackmoor civilization use it, and starting from a little province in the outskirt of the Thonian empire, it first gains its independence, then grows and finally comes to rule over the adjoining cultures.
I take this from official sources, Wrath of the Immortals above all, and this goes on until Ogdoban Treel wakes up from his induced torpor and choose to do what forced Benekander to act towards the Beagle annihilation.
Was it the end of techno-magic? No, not at all, I should add.
The sources are pretty clear and above all coherent between themselves, but following the stories in the four module adventures from the DA series and in the aforementioned Wrath of the Immortals, I think that the more likely chain of events is something like this.
The ship falls down and from that point on (DA2 and DA3) there are contacts between the Blackmoor Realm and the ship’s inhabitants (the so called Gods).
There are few meetings and exchange of gifts and alien items. Said items diffusion is very limited. Perhaps the King’s Companions (Uther Andahar, his cohorts and the Blackmoor University) got their hands on them, and by logic a few of them fell into the hands of some adventurers.
What magic-scientific developments may have take place in those years (the King Uther era, to be clear)?
From my point of view, that is nothing worth truly subvert a campaign with a definite fantasy connotation. Maybe some lightsabers, some exotic weapons, grenades, some high-tech gizmo (a chronometer, a transmitter, a flying suit, or a jumping suit or a diving one) but still nothing able to stray the setting to the sci-fi genre.
Years goes by, maybe centuries, during which it’s possible that Blackmoor University starts manufacturing in large scale one or more of said item, to make them becoming more common and easy to obtain, especially for their own armies. These are the years during which the Realm’s wizards learn to conjoin the tech they inherited with the magic they already know.
It’s possible that the main event of this period is connected with Treel the alien’s awakening and with an unlimited access to the Beagle’s treasures by the Blackmoor scientists (a more apt title for this new kind of tech-wizards).
When the alien ship actually implodes it’s clearly too late. A big part of her treasures has found its way in the various research centers scattered in the Blackmoor Empire.
How much time from this point to the Great Rain of Fire? In Wrath of the Immortals it says a millennium, but I for myself tend not to believe it. Reading other official source materials, it seems more than plausible that the time span, from the Beagle’s arrival to the holocaust, is five centuries long.
One, maybe two are the centuries during which Blackmoor accomplishes its technological development at its fullest, reaching the top of its scientific evolution, conquering the whole planet and then …. puff…. disappears.
While this all happens, what the Egg of Coot is doing?
If you are curious you will find the answer in the next post: The Return of the Egg – second part…coming 15 December
 Series DA modules are: DA1 – Adventures in Blackmoor; DA2 – Temple of the Frog; DA3 – City of the Gods written by Dave L. Arneson and David J. Ritchie and DA4 – The Duchy of Ten written by Ritchie alone.