And they'll likely only read parts of it

The Third Civilization is the first time sapient organic life has evolved in this multiverse. The First Civilization came from another multiverse 30 million years in the past and left ~12 million years ago to another multiverse.

The Second Civilization was made of pure spiritual energy, evolved from the echoes of the First Civilization's souls around 9 million years ago (from the game's perspective); they could only manipulate the physical realm by possessing some animals. (Their favorite targets were simian creatures because of their useful limbs.) ~5 MY ago they found the natural pathways to the outer and inner planes plus the Feywild and Shadowfell-equivalents where their spiritual energy could embody without having to possess creatures and left. Currently they are known as gods and high-ranking celestials, archfiends, archfey, and the highest of the primordials.

And then ~2 MY ago those simian creatures that they liked possessing started evolving true sapience and sentience, climbed down from the trees and started walking on two legs, etc. Some groups of these beings (basically Homo Erectus) even made their way into the more friendly inner planes (the plane of Earth and the plane of Water), the Feywild-equivalent, and even the Shadowfell-equivalent.

As their society and culture evolved, so did the complexity of their brains (also spurred on by the use of fire which started on the Material Plane by the group that would evolve into Homo Sapiens Sapiens). The Second Civilization entities noticed that after death their more complex neural patterns didn't dissipate but instead passed on into the outer planes they inhabited, empowering them enough to affect changes on the Material Plane.

This sparked the Soul War, ~500 000 years ago. The Second Civilization entities on the Outer Planes were divided on how they should proceed with these new creatures. Many saw them as creatures similar to the First Civilization whose spark of sentience eventually led to the evolution of the Second Civilization, and as such they considered them peers worthy of respect (or at least free-willed creatures that might end up making good allies or powerful servants). Others just saw them as a convenient power source that needs to be captured and harnessed.

This ideological split led to the groups currently known as the Gods and the Fiends. The Primordials and the Fey were initially neutral in the conflict. The war raged on for 300 000 years, using the Material Plane as the battlefield and many of the early hominids as pawns. When it rippled into the inner planes, the Primordials and the Fey tried to play peacemaker but the differences between the gods and fiends seemed irreconcilable. After long debate they joined the war on the side of the gods.

In response the fiends started developing weapons capable of massive multiplanar destruction. The gods were willing to fight to the bitter end, but the Fey-Primordial alliance offered a compromise that was acceptable to both parties. The war ended with the victory of the gods but the fiends were allowed to build three thrones for themselves, and those who sit on the thrones would have the powers (and responsibilities, and limitations) of a god. (These were the Throne of Tyranny, the Throne of Corruption, and the Throne of Destruction.)

This was also when the first Divine Concordat was signed, severely limiting the direct contact between the outer planes and the material plane, and limiting the powers of fiends and celestials. (Elementals and Fey got a special exception since their planes are more closely-linked; they are capable of entering the Material Plane uninvited but their power there is only on the level of a mortal.)

Part of the Concordat was also the reconstruction of the Material Plane and the inner planes; gods, fey, primordials and even fiends were allowed to create new sapient races (with an ever-evolving set of limitations, e.g. after the creation of dragons they limited the maximal size and lifespan of the new races, and so on). This is the start of the Third Civilization in its earnest.

For ~150 000 years most races lived as primitive hunter-gatherers. But giants who evolved to colonize some of the more inhospitable regions (that were also affected by the magical fallout of the Soul War) had much longer lifespans, and this helped them develop their magic and technology. When they saw that the smaller races also started developing technology similar to their own, they knew that they wouldn't stop, so the Cloud Council decided to conquer them in order to prevent them from becoming a threat. This started the Age of Giant Kings ~50 000 years ago, with most humanoid settlements on the Material plane and the elemental planes being ruled by a fire giant or ice giant in the name of their cloud giant emperors.

This didn't last too long however. The cloud giants required tribute, and the fire and ice giants, wanting to curry favor with the cloud giants began teaching magic and technology to the small ones. This was forbidden by the cloud giants, per se, but they ruled so distantly that this was rarely enforced. The small ones organized resistance cells and began teaching these technologies and magic to other settlements whose giant king didn't do so. Then ~30 000 years ago the organized resistance achieved their first success, slaying one of the cloud giant emperors who ruled over the continent that was the ancestral homeland of the humans.

The next couple of decades were a series of rebellions, ending with 90% of the giants dead. The smaller races were aided by the fey, some giants who were sympathetic to their cause (mostly fire and ice giants), and some rumors say that even the gods helped them since the giant rule was uncomfortably similar to what the fiends wanted.

In the end the Cloud Giant High Emperor enacted the Curse of Forgetting on the world: everything that the giants knew would be forgotten and nobody would be able to re-learn it for 100 generations. He warded his own palace and close circle of advisors but unbeknownst to him one of the advisors was a rebel and he sabotaged the wards, leading to not only the smaller races but also the giants forgetting their magic and technology. What the High Emperor didn't account for was that the smaller races had their own homegrown technology and magic too, so the Curse of Forgetting completely backfired, making the smaller races the dominant people on the planet. This ended the Great Giant Civilization overnight, with the remaining giants retreating to the elemental planes or the most unhospitable reaches of the world. With Giant lifespans being much longer than those of the smaller races, their 100-generation curse also lasted much longer; they only began reclaiming their knowledge well into the life of the Fourth Civilization.

The Antique Period of the Third Civilization started with the First Bronze Age, during which people were unable to learn iron working or stonecutting, those being technologies that the Giants taught them. So this age was dominated by small settlements of wood, clay, and bronze, with bronze working, woodworking, and pottery being technologies they knew even before the giant conquest.

3000 years later the shorter-lived races suddenly found themselves able to invent ways to cut stone into shapes more complex than axe-heads and knives, and even smelt that elusive iron. The longer-lived races followed in a few millennia. For the next 3500 years this civilization developed similarly to civilization on Earth, but their reliance on magic led to some differences (i.e. they never discovered gunpowder or steam power; that is a technology unique to the Fourth Civilization).

At the end of this period the planet was dominated by three superpowers with ever-shifting alliances between them, locked in a cold war.

This cold war heated up at the end of that period, and the use of powerful arcanotech weapons led to an increase of volcanic activity which pushed the planet into a centuries-long volcanic winter. Crops failed, trade faltered, and starvation caused a population collapse. Some technology was preserved in religious and secular institutions but the surviving population was reduced to iron age levels. It took the climate millennia to stabilize and allow the population to start increasing again.