3.
There are no NPCs within the Decagon at all. However, there is a system that is located somewhere, where if active, the demons can go about winning this game, as it would wipe out all players instantly, allowing the demons to automatically win as a result of this. This system is something that they have to discover for themselves, as it is not spelled out to them obviously. Though there are hints that lead them on to the discovery of this system that exists for them alone. It was something that they had to fully discover on their own, as they went about carrying out their role as a demon. How that works is up to them, as they are not told how to act as a demon at all. That is all up to them, as even though they are now demons, they are still players themselves at the end of the day. They are a unique type, where there are only nine of them; however, they are still at their core players. The divide between them and the ten thousand other players is the kind of player they are rather than what they are. Playing a boss in a game is considered normal in some aspects and genres. However, how many of those games allow a player to actually play the superboss, though, not just merely the final boss? The final boss is not even normally the true skill check of any game itself. They are merely the end of the story itself, as sometimes they are not even that difficult to handle, and other times their strength can be easily overcome with enough effort and time, as they can not improve beyond what they are already. Whatever form they have or take is capped at a lower limit than that of a superboss after all.
This system becomes known as the tenth demon by the demons that discover it. As the first clue is in how there are so many mentions of the number ten over and over.
Ten thousand players
Ten thousand rooms
Ten different facility areas
Ten Starting areas
Ten sets of one thousand players
Ten branches of the Guild
Ten sided shape, the game itself is named after
Many other things that have the number ten involved, but there are only nine demons. The one area where nine is used instead of ten is that of a fluke, or hiding something from them in plain sight. The idea of there being a tenth demon is written off by many of the demons, as they were told that there are only ten thousand and nine players by Kynatech and the developers repeatedly. They were very specific and honest about everything they said and did. Too honest, to the point where they were able to hide numerous things from them, as their honesty was so much that they overlooked a lot of things that overtook them, which is why they never made it to become a beta test. Seeing through a lie might be easier than seeing through the truth. As telling the truth does not always mean explaining what you fully mean, as the one who does not ask follow up questions can be blamed for not doing so. Telling the truth and explaining it are not the same thing. Expecting someone to do so is an expectation, not a requirement after all.
Thus, some of the demons learn something through seeing through the what is presented to them, that the tenth demon is not a player but a system within the game that they have to discover. Awaken that system, and they can easily win the game. The biggest problem here is that there is no guide to finding it at all....easily, at least.
3b.
Within the third problem is something that works to the demon's favor. As players are given the freedom to do whatever they want within the confines of The Decagon. They are allowed to act or not, play or not, or even be actively present or not. However, the one thing that many players overlook is the option to join the demons or not. If the option to join the other side exists, then why would it not be a choice that could be made within this game? That reasoning, that players would never join the demon's side when their goal is to kill them, is how many of the players overlook something that is right in front of their face once again.
The path of joining the demon's side was never not an option; it was always on, be it official or unofficial. Some groups merely proclaimed they were followers of the demons, but those who just follow the mere idea of the demons themselves are the unofficial ones. Just like the first demon, no one who is a demon follower openly goes around stating the fact. That would make as much sense as someone who goes around yelling their full name, title, and status to people they have just met for the first time in life, be it in person or through some other means. Those who are true demon followers are not followers of the mere idea of the demons themselves, but a specific one. Thus, have pledged their loyalty and service to that one particular demon, becoming their follower. Even their own followers are not normally allowed to know who their demon really is, as they see their demon forms (they have two different ones) if they are lucky and truly considered worthy. Seeing their player form is normally only done to a select chosen few who have proven their worth.
Betrayal can still happen even when the player is a demon follower, however the price to pay to become a demon follower is the handover of their soul to their demon. Thus, they tie their soul to that of their demons, so that if their demon dies, that means they follow them, as their soul is tied to them as well. This does not affect the count of total players, though, as their souls are not tied to their lives in this game. This was done to prevent the cheese method of mutual destruction from being picked, as the developers and Kynatech were not allowing this beta test to end in a tie. Splitting the soul from one's life prevents those tactics. Getting all the players to pledge loyalty to a demon, then working endlessly together to kill that demon. So what happens when all nine are dead, all the players are too, is avoided.
However, this itself was not even a possibility to start with, even if that were true, because of the fourth unique problem, known only to demons and their most trusted ones.
(Note: The first demon to fall had no official followers because they were not aware of this possibility, and no other demon told them.)