Demons, in general, are very bad at cooperating even though they all share the same basic goals. One of those goals is promoting mistrust, and betraying people is one very easy way to do that.

With an awful lot of effort, and hundreds of years of preparation, Slzlth was able to assemble a team of demons that might just be able to hold together long enough to pull off killing God. Apart from having personalities that made them at least minimally capable of cooperation, the other demons were also selected for their particular talents: Thrl for their ability to get humans to like them through genuine empathy, Pfmsltr for their hex of sanctuary and general strength, and Pns for their hex of damnation. He arranged for himself a perfect human cover by living as a human, Sebastian Marlowe, from birth, while Thrl and Pfmsltr made themselves somewhat simpler human cover stories and Pns remained in Hell. There were probably various other demons involved in minor ways. Tslt was not part of the plan, they just happened to also be living near Bexgate at the same time.

After all possible preparations had been made that didn't require such a high level of action from humans, and an opportunity arose, the demons accepted the necessary reduction in secrecy and took the next steps. When an irregularity occurred in the transfer of control of the sanctuary at St. Renagi's from Ague Vitis to Jamie Holton, the demons took advantage of the opportunity to place a hex of sanctuary on the same building. This was what prevented Heaven (and to some extent also other humans) from seeing in, and was the reason for the butterfly mark on the wall outside.

Next, they needed to open a portal to the spiritual plane that they could send witches through later to kill God. The ritual to do this required the various items the demons asked for. Pfmsltr's ability to see the emotional significance attached to objects was also important in this part of the plan.

The items were

• A mortar and pestle
• Fenland honey
• A token of new friendship
• Something that made someone cry
• The name of a forgotten person
• A useless tool
• Bottled rage
• A dangerous precaution
• A promise kept by breaking another
• A confession of love
• A deathbed item
• A confession of murder
• And most importantly of all, a saint: St. Renagi, provided unwittingly by all the people at the group prayer action.

Using the extra magical power gained from all of the other items, the demons were able to damn St. Renagi, despite her already being a saint. The first time they tried the ritual they were still missing a few items but they had to try anyway because Renagi would have removed the sanctuary otherwise, so they partially failed which is why she ended up frozen in the church for a week. The demon many people saw performing the ritual but didn't recognise was Slzlth, using an illusion to cover up his normal appearance. Shortly after that, they collected all of the remaining items, arranged for the Thackeries to be distracted, and succeeded in damning St. Renagi. With a double connection to the spiritual plane, Renagi was unable to avoid returning there, and her hair gained the same property because it was a part of her. The demons mixed the hair with wax so that the mundane matter would resist the hair's pull to the spiritual plane, then made this into the wax cubes they later gave out. There was also a minor illusion attached to all the cubes so that Heaven couldn't find and take away the many backups.

Now that they had the ability to get people into Heaven, the next step was to get people to go there. Demons couldn't go to Heaven directly themselves, they needed humans to do so. They at this point asked the witches for two things, to steal the weapons from Heaven which would be capable of killing God, and to connect Heaven and Hell so that demons could attack God, serving as a distraction and possibly weakening, if not God Themself, then the angels defending Them. The second part of this went to plan, but the first didn't, since Heaven had moved the weapons to be stored more securely by the point anyone tried to steal them.

With Heaven and Hell mixing together and God under attack from almost all of the demons and many of the damned too, God was forced to act to divide Heaven and Hell more permanently. The side effect of this was the physical plane (or at least the vicinity of the Earth) was also divided in two, one side connected only to Heaven and one only to Hell. All the angels and saints got sent to Heaven, all the demons and damned to Hell, and all the other things in the spiritual plane which weren't people (including the weapons) got divided at random between the two. St. Renagi, being both damned and a saint, got stuck in her own little partition away from both Heaven and Hell. From this point on, it would no longer be possible to connect Heaven and Hell like happened the first time because God had created a much stronger separation between them. Also, the demons and damned could only manifest on one side of the rift, and angels and saints could only manifest on the other. Hexes and miracles also couldn't pass the border, and from Heaven and Hell, it was only possible to see into and hear prayers from the corresponding side. Heaven did not want anyone to be away from them and subject to Hell's uncontested influence, which is why Tabaga ordered everyone to evacuate to Heaven's side.

With half of the spiritual weapons now in Hell, Pns was able to collect quite a few of them. The demons in general were initially excited to find them, only to discover that they couldn't use them themselves. Without portals (which only the plot-associated demons had access to), they didn't even have any way to give the weapons to witches, but Pns did. The next turn, the witches of St. Renagi's (who had expanded considerably in number by this point) used the weapons to attack God. I'll leave the details of how each of them was involved in that up to them to reveal during debrief.

With God dead, the demons no longer had any incentive to cooperate. Slzlth at that point didn't care, the job was done, and just let them fall apart. Thrl genuinely caring about the humans and Pfmsltr delighting in hurting them led to particular conflict between those two, with Thrl participating in killing Pfmsltr.

After God separated Heaven and Hell, creating more portals became most likely impossible, making the existing ones extra important. In order to have control of as many as possible just in case they were needed ever again, and in order to avoid the witches of St. Renagi's from getting to Hell and killing any more demons, Slzlth then collected all of the portals that had been handed out and not already captured by Heaven (unless I forgot one OC) during the last email period, and was even able to get the witches, who at that point all felt they were done with Heaven and Hell, to hand them over willingly.

The main reason why Theodore and Uriel were damned was to try to prove to the parishioners that Hell is not as evil as it's made out to be. After all, if the damned are just people like Theodore who isn't scary or hurting at all, what else have the Commission been lying to you about? And of course OC it was a convenient way to allow characters to remain playable even after being executed.