It begins by showing a crossover, and how both series' characters meet in Heaven again, this time because God, the character attatched wants to show a big presentation or something. If you want to be very Hazbinesque, call it a musical number. (Not making it a thumbnail because you got to see it in full glory, lol). This can be expanded upon so the cream of the crop of Hell gets to see: Charlie, Lucifer, Alastor, Paimon, Stolas, etc.
Then, something happens mid way, and a grandiose light shines through, and a great revelation comes. Said ray of light teleports seven (or thirteen: one plus twelve, making due to the Twelve Tribes of Israel) Knights akin to the Templar Knights we see in Doom Eternal (or at least inspired by them) that come through. They stand in place, while said light form speaks, and reveals that said "God" of Hazbin Hotel is a false god in
similar concept, albeit original fashion to the whole
gnostic concept of God: Demiurge. The True God being the "Demiurge" while the false, silly god being just a... uh, fujoshi thing that promised said True God he would play along and go with this world to do things. Finally, the knights would then reveal to be the 12 Apostles & Jesus Christ, revealing Saint Peter of Hazbin Hotel to be a fake as well.
These are the Night Sentinels from Doom 2016 + Doom Eternal - inspired by Knights' Templar armor (and also my inspiration from the Apostles & Christ's armor).
"I have come to do the work of My Father" announces Jesus, and they do an entire Doom Slayer thing, killing the denizens of Hell with incredible ease - but ALSO killing everyone in Heaven, taking hostages too - Especially Satan, which would be tortured or even killed. The false god then decided to shed some power to save some of his host of angels (don't know some of them, but I want to include Adam here), as well as denizens of Hell to be spared. You then would get something that wouldn't look out of place from a Mad Max/Fallout , with the said surviving cast finding what's left of Hell living akin to an atom bomb/TEOTWAWKI (The End Of The World As We Know It). That means, demons, imps, angels and everything else living humble lives, eating from scraps, and such effects even hitting on the humans on Earth, as the events of Revelation shall unfold and Humanity must pay the price of their sins.
And of course, you'd see beloved characters such as Nifty, Moxxie, Millie and even Loona die or have sacrifice them sacrifice in order to have their friends escape. After all -
Greater love has no one than this: to lay down one’s life for one’s friends. (John 15:13). This could be an example of the many of the characters justifying salvation for themselves or at least ensuing a less bad ending. This raises the stakes and makes it just as Vivienne Medranno saying that she isn't afraid of killing characters.
Through Trials and Tribulations, eventually (I didn't plan this outright), you'd get to see the Apostles planning on a "Heaven on Earth" scenario, and Jesus Christ judging each person according to their works, perfectly and without a fault. The end confrontation however would be a stalemate. The goals of the Apostles and Christ were righteous, and there was nothing to stop them. However, they have a chance to start from zero, and rebuild Hell from scratch, so it can become a place were either people go to suffer in torment for eternity (which is Christ's plan) or convince him that sinners can redeem - which Adam will play a huge role.
It will show eventually that each denizen located in specific circle of Hell (Pride, Lust, Gluttony, Greed, etc) that managed to let go of its specific sin and helped the "main cast" in their tribulation, managed to attain a "mark" which could be a redemptive mark. In the end, Jesus would reveal it was in His plan to redeem sinners His own way, and cleanse sinners, albeit in a brutal way. Maybe we could shoehorn Satan breaking free, angry that "his" plan was foiled, and something like a "reduced" version of Revelation 20 happens. Satan absorbed all the unrepentant souls and grows in power and is completely stronger, and Adam completely redeems himself by giving away his life to give the Apostles a fighting chance, (maybe the false god too would do that). In the end, Jesus Christ becomes victorious.
And that's for the 1st version.