pg163-164
The "Wall of Fire" was written to exist in the "Digimon Book of Revelations," but its existence was thought by Digimon to be symbolic rather than realistic. Only Gennai seems to know it to be real. The Wall of Fire was able to appear in the Adventure series because of the formation of Spiral Mountain; while the digital world was under the protection of the 4 Holy Beasts, the Wall of Fire was kept hidden deep underground, but with the formation of Spiral Mountain, all of the world layer above it got sucked up, leaving it fully visible and its fire weakened. Gennai explains that Apocalymon has escaped through the Wall of Fire and Apocalymon isn't exactly "one being," but rather a concept (an "idea") that had been rejected by the digital world, to make the digital world what it is now.
pg206-208
Gennai and the other Agents who work for Homeostasis are the ones who made the children's crests, under Homeostasis' guidance. The crests were made one by one, with their hopes that those who bore these crests would save the digital world. Apocalymon's initial appearance to the children (in the novel) is more like that of a beast, destroying these children's crests with a single cry of rage.
Long ago, Piemon inserted a Seed of Darkness into Gennai's back, and when Gennai feels despair over the loss of the crests, the Seed gives him sharp pain. Gennai has to fight with himself to control his emotions, because the Seed is activated by negativity. This is also the reason why Gennai lived underneath Temp Lake with a barrier around his residence, so as to keep his mind at peace and discipline himself with a constant zen attitude.
pg213-219
When Apocalymon finally speaks to the children, it refers to itself as "we" and has a voice made up of multiple voices. It speaks to the children with the words of all the digimon it had sucked up, who had failed to succeed in evolving and surviving. Homeostasis explains that Apocalymon is really a "concept," not the physical body that they see in front of it. It is a concept that rejects evolution, one that refuses the option of evolving. When the digimon that failed to succeed died, they are reborn in the Village of Beginnings as a new being, but their feelings of regret remained and were collected by Apocalymon so that it could manifest its physical shape. Now that Apocalymon has achieved physical form, it is accumulating energy to unleash a Big Bang that will destroy the current digital world.
It's also interesting to note that Takeru is the only one who never receives his crest, in the novel, and thus was not all that shaken up when Apocalymon destroys them. Homeostasis explains that the crests are really amplifiers of the "spiritual" special qualities that are already inside each of the children; it's a helpful mechanism but they do not actually need those amplifiers to evolve their digimon. Takeru evolves Angemon to HolyAngemon the first time without a crest, simply by the strong amount of hope he had that they could win, and this "lights" the way to stimulating the others into activating the crests that are inside their hearts. Being able to not only feel their special quality but see it appear in physical form helps the children to understand that everyone's individual trait inspires and is inspired by other people. They can't do this fight alone.
pg221-222
Since Apocalymon is a concept, it does not need its physical body and therefore doesn't care too much when it is attacked. It prepares to unleash a big bang, then reconstruct a new stagnant world where it would rule as a god, worshipping only bleakness and death. Apocalymon's physical form disappears to become an area of cold and complete darkness. The children's determination that they still have a future activates their digivices to create a barrier that completely enfolds the big bang, shrinking until it disappears.
pg227-234
At the beginning of the world, when the digital world was still in a state of chaos, the digital world decided on the concept of "evolution" upon its formation. In order to follow-up with this concept, the Homeostasis was born. But because Homeostasis is a security system without a physical body, it created independent Agents like Gennai to act as its physical laborers. Light and darkness are two sides of the same coin, and the world is at balance when light and darkness are in equilibrium. Homeostasis is a system that must maintain that balance, so when the power of darkness amplifies excessively, it must do whatever it takes to prevent it from destroying the balance of the digital world.
Then, one day, a digital egg accidentally crossed the gate between the digital and real worlds, landing in Hikarigaoka and hatching to evolve from Koromon, Agumon, to Greymon at incredible speed. This speed of evolution was unthinkable in the digital world. The cause of such a rapid evolution was a girl named Yagami Hikari. Hikari possessed something that accelerated evolution, but Homeostasis did not understand what that was and sent Parrotmon to retrieve Greymon. As that was happening, Homeostasis scanned Hikari's data as well as the data of people around her, including her brother Taichi, for comparison. The Agents analyzed this data and discovered that certain children possessed special qualities of courage, friendship, love, knowledge, purity, sincerity, and kindness-- not as qualities that they already had or didn't have per se, but qualities that contained a hidden potential within them. However, what Hikari possessed wasn't anything like those traits; it was more like the power of "life." They (Homeostasis and the Agents) found this power to have the same meaning as "evolution" did in the digital world, as well as the meaning of "truth" and "beauty." In order to represent this power as "the source of life," they named this special quality "light."
A single year in the human world means several hundred years in the digital world, so the digimon continued to evolve during this time, and eventually there appeared digimon who appeared to have a connection with the special traits of humans in the real world. These digimon were able to achieve accelerated evolution through the power of that child's special trait, and drive out the power of darkness in the digital world when it got too great. Those digimon grew to be called "partners." Homeostasis created the digivice in order to relay the human's feelings to their digimon. It also created the crests, and each crest design is actually what it appears from within each child's heart as an oscillograph. The crests worked not only as an amplifier, but also as a limiter if the human's special quality was motivated by evil intent.
This was how the digital world overcame its dangers for many years. This also was not the first time that Apocalymon appeared; the first time that it did, it was successfully trapped by the children's powers and digimon, but there wasn't enough of both children and digimon in numbers to seal Apocalymon away completely. Four of these digimon evolved into the Holy Beasts and gained the role of protecting the digital world. The events that took place with these Digimon were recorded in the "Digimon Book of Revelations" for future generations.
The Dark Masters, plotting to take over the digital world, got their eye on the Book of Revelations and used it to set up an evil plot, using Nanomon to create a program that would form Spiral Mountain. While that program was being created, the four of them attacked the Four Holy Beasts to seal them away. It is surmised that Apocalymon sent the Dark Masters its "concept" across the Wall of Fire, because the Dark Masters' plan, without them realizing it, would utterly destroy the Village of Beginnings' function. In other words, creating Spiral Mountain stole the concept of "evolution" from the digital world. Unconsciously, the Dark Masters were manipulated by Apocalymon into rejecting the idea of evolution.
Homeostasis kept track of Hikari, Taichi, and the other children when they moved out of Hikarigaoka into (coincidentally?) Odaiba. Homeostasis also used its Agents to pick out digieggs from the Village of Beginnings that had the potential to become partner Digimon with those children and bring them in for safekeeping. However, Piemon somehow found this out and attacked the Agents. No Agents except one survived, and this was Gennai. Before Gennai ran away with the digivices and digieggs, Piemon buried a solid ball of darkness into him so that, even if Gennai escaped from him, it would prevent Gennai from moving around freely. This ball of darkness would later be reformed into the Seed of Darkness and used by dark digimon as a weapon.
Gennai (although he unwittingly dropped Tailmon's digiegg), shook off his pursuers and hid the digieggs in File Island in an area that was still covered by thick glaciers, before diving into Temp Lake to hide. He built a barrier beneath Temp Lake to prevent the ball of darkness from activating, and thought of what he could do to limit the ball of darkness's power. What he came up with was to "age." By lowering his regenerative function, he would be able to prevent the ball of darkness from enlarging.
The digieggs on File Island hatched, and the partner Digimon waited for their humans to arrive. The glaciers melted and greenery sprang up as they did. When there began to show signs that the Holy Beasts were wearing down and the Dark Masters plans were looking more and more successful, they sent one of their minions, Devimon, to File Island in order to spread darkness throughout the digital world. As the power of darkness grew, it upset the balance of the digital world and the digivices flew to the real world to summon the chosen children.
