The Altarian Prophecy was an expansion pack released for the original Galactic Civilizations in 2004.
It was the first attempt to introduce players to the back story to Galactic Civilizations II. It introduced players to some of the great powers from long ago:
First there were the five Mithrilar. These were immortal beings that long ago existed and have long since disappeared. There were two in particular -- Mascrinthus and Draginol. (Draginol and Mascrinthus are actually the screennames of the lead programmers for the game.) Originally there were only four, but then Draginol appeared. According to him, his universe had just imploded in on itself. The others thought he was different in some way, but they did not know how. Because of this, he was called the Dark Mithrilar.
The reason for this was simple: Draginol was the only Mithrilar aware of the passage of time. This had driven him insane a long time ago, but still sane enough to be aware of the source of his pain.
By abusing a device called the Telenath ,Draginol created a race called the Arnor who are known to most civilizations as "The Precursors". The Arnor eventually split off into civil war with the side in favor of exterminating new sentients as they evolved as "Dread Lords". According to the manual, the Dread Lords are closest to Draginol in character, but have smaller numbers, as well as a sense of time perception. The Arnor, on the other hand, are more numerous and have no time perception.(Draginol didn't spend as much effort on them.)
The question that naturally arrises is how the Arnor were able to run amok in the galaxy if the Mithrilar, who were infinitely more powerful, were at hand. The answer is, they weren't. The Mithrilar disappeared in a cataclysm. The only two that survived were Draginol and Mascrinthus.
According to Altarian legend, the dark Mithrilar, Draginol, had fallen to a then dead world called Altaria and for millions of years been buried. His presence, even unconscious, altered the evolution of the Altarians such that there were two sentient races competing for the world.
One became known as the Drath. These reptilian like creatures were strong but reproduced at a much slower pace than the human-like species that we now know as Altarians. The Drath somehow managed to escape Altaria long before it had space travel. We do not know how they managed to do this, although the Mithrilar Mascrinthus might have something to do with this.
The Altarian Prophecy hence implies that Altarians look like humans due to the influence of dark Mithrilar, Draginol. The question is, why would that be the case given that Draginol's arrival on Altaria was millions of years before humans had even begun evolving on Earth. That question remains to be seen.
More troubling is the arrival of the Thalan who claim to be from another dimension and claim the humans are going to do something that will destroy the universe one day. But other than that unhelpful cryptic claim, how it relates to all this is still unknown.
According to Mascrinthus(the programmer,not the Mithrilar), DL Bradley, one of the humans that explores the Dread Lords' pocket universe learns all their technologies and secret of their creation , eventually became Draginol and time traveled to the Mithrilers time, causing an infinite loop sequence of events that the Thalans know about and are trying to break.