So, I have a big plotline I'm putting together for the campaign I'm on, as a baseline for the players to screw up (I mean, this is the BBEG's plot so yeah, it's the PC's job!).

Long story short, I am setting up the Raven Queen to initiate Ragnarok and use it to sieze power from the norse pantheon, essentially cementing herself as a monotheistic deity.

I have a plot for how she starts it, a plot for how Fenrir and Jormungand are released, a plot for the slaying of Baldur (which begins the Fimbulwinter) and heer plans to take Ysgard when ragnarok happens. I have a plot for why the Giants would march on Ysgard, as per the tale of Ragnarok, but I am drawing a blank for why the Demons would march there as well.

The plot I am forming is, hopefully, going to be the old switcheroo where they spend the first half of the game inadvertently causing this to happen, and the second half trying to stop it. As such, the events which cause the Demons to march on Ysgard need to be initiated some years before they actually march.

The Raven Queen being the goddess of fate, I intend for her to be pushing things the way she wants them to go, so I need a decently broad idea for why the demons would march to war; EG so-and-so is released, or whatserface is slain, leading to a chain of events which culminate in the armies of the abyss marching on Ysgard.

And that's where I'm stumped! I need something for the party to do which will set this in motion, but I don't know enough of the motivations of the abyss and their residents to properly formulate something!

• Party goes into abyss -> Kills powerful/important demon lord -> Next demon becomes king (might call in favors from other groups of demons to get them to help with the war) and seeks revenge against the characters -> The demons decide they will obliterate the characters and all allies and gods who protected them and whom they allied with.
• Party messes with a powerful group of demons hugely important plans to conquer the above world, defeat the devils, etc. -> The demons decide they will obliterate the characters and all allies and gods who protected them and whom they allied with.
• The Raven Queen tells the demons about how the above worlders plan to destroy them all and steal their land on orders from their gods -> The Raven Queen then goes around from important above worlder city to above worlder city and uses magic, shapechanging, and trickery to gets dozens of dozens of super-strong adventuring parties to be sent into the abyss to kill demons -> Most parties don't survive, but the characters are one of the few to escape after killing numerous demons and angering their lords -> Raven Queen tells the demons "See, I told you guys that all the gods and humans and elves above are trying to destroy all. They will send more and more murderers down here if you don't deal with them." BTW, you could also bring in some other people from parties that went into the abyss and survived as NPC's. Some of those NPC's might have been cruel and done horrible things to the party

Demons don't need good reasons to hate the above world. They are spiteful and evil creatures who already hate everyone who is different than them. The demons don't need a good reason to join Ragnarak, the players would just be the last straw that broke the camels back.

Also, do note that the players might need to start out at higher-levels than usual if you want them to fight demons immediately. And players don't like feeling rail-roaded, so if your players do a great job and avoid upsetting the demons, then let them. There is no point in making it be an important part of the plot that the players got the demons to join Ragnarak if they had no way of not doing that.

I don’t think of demons as marching, per se, that’s what devils do. Demons are just crush, kill, destroy. If one accepts that premise, they don’t need a reason, they just need an opportunity. Open a portal between wherever the demons are, and where you want them to go, and let nature take its course. 

Thank you both for your responses!

BoringBard, I like the idea of the Raven Queen getting adventuring parties to kill demons. The party is currently at level 9 and I'm planning this as a big story arc for them to go from 10 to 20ish, so I have time to get them to level up a bit more!

I am now thinking that I shall plant a champions guild dedicated to Odin and the Norse Pantheon, and have the Raven Queen manipulating it from the background to get them to kill demons. I may throw clues, such as the guild tackling  a new threat of demonic incursion and the gate actually being incredibly old and just disused, and such for the party to unpick. 

The raven queen won't hedge all her bets on the party getting the job done, so I will try to find ways for them to provoke the demons as they go further, and make a backup plan not involving them should it be needed. In fairness, their goal in the end is to prevent it, so if they don't provike the demons, the demons aren't provoked.

Rail roading the players is the last thing on my mind, I'm just trying to work out plot hooks that will tempt them but which might incur such side-effects if they succeed! Ideally indirectly - for example, the current plot line involves slaying a dragon. When they do, the thunderstorms around its lair end, the river softens, and the lake by the town will start to recede, ultimately revealing a temple to the snake god (which may be because I have the Epic Encounters box to play with), which will awaken the cult that will release Jormungand. Meanwhile, the warlock will be told to pick up something from the dragons horde - first causing the dragon to give chase and then secondly to be revealed as the phylactery of a lich, who will start to attack the party (seemingly for no reason, they don't know what the thing was) and become the BBEG of the second arc, which will culminate in the start of the Fimbulwinter. I'm trying to plan it all around key points in the game and leaving the rest of it - such as where, when, and how - up to the party!

Quote from ThorukDuckSlayer >>

Thank you both for your responses!

BoringBard, I like the idea of the Raven Queen getting adventuring parties to kill demons. The party is currently at level 9 and I'm planning this as a big story arc for them to go from 10 to 20ish, so I have time to get them to level up a bit more!

I am now thinking that I shall plant a champions guild dedicated to Odin and the Norse Pantheon, and have the Raven Queen manipulating it from the background to get them to kill demons. I may throw clues, such as the guild tackling  a new threat of demonic incursion and the gate actually being incredibly old and just disused, and such for the party to unpick. 

The raven queen won't hedge all her bets on the party getting the job done, so I will try to find ways for them to provoke the demons as they go further, and make a backup plan not involving them should it be needed. In fairness, their goal in the end is to prevent it, so if they don't provike the demons, the demons aren't provoked.

All good ideas:) I'm so glad I could help!

Quote from ThorukDuckSlayer >>

Rail roading the players is the last thing on my mind, I'm just trying to work out plot hooks that will tempt them but which might incur such side-effects if they succeed! Ideally indirectly - for example, the current plot line involves slaying a dragon. When they do, the thunderstorms around its lair end, the river softens, and the lake by the town will start to recede, ultimately revealing a temple to the snake god (which may be because I have the Epic Encounters box to play with), which will awaken the cult that will release Jormungand. Meanwhile, the warlock will be told to pick up something from the dragons horde - first causing the dragon to give chase and then secondly to be revealed as the phylactery of a lich, who will start to attack the party (seemingly for no reason, they don't know what the thing was) and become the BBEG of the second arc, which will culminate in the start of the Fimbulwinter. I'm trying to plan it all around key points in the game and leaving the rest of it - such as where, when, and how - up to the party!

My bad, I misunderstood you: I originally thought that you were saying that the players had to be responsible for the Demons marching to war, and that their actions must be the things to spur it. My mistake for misunderstanding what you were saying.

A common trope is killing an enemy releases a greater evil

• The blood of the world serpent could spout demons from it
• Killing the god of hell could unleash demons that they held at bay 

Another option is trickery

• Loki or Odin could offer assistance in some form with part of heir plan being releasing demons. Both are commonly trickster figures. 
• The raven queen herself could also appear to offer help possibly in the form of some one else.

A common trope for attracting aliens which includes demons as a kind of dimensional alien is having some kind of artifact or resource they're attracted to

• some one could steal something from the demons 
• an ancient relic could be reactivated 
• the demons may want some power on the earth that some one claims is there