My Homebrew Game based loosely on SKT

After running 5 brand new D&D players through CoS, they wanted to keep playing and I wanted to run a homebrew game in my own world.  I really like the story in SKT so I stole some of it as part of the overarching campaign premise.  The story boils down to this; an ancient red dragon that has been plotting for a long time is finally ready to begin her attack on the kingdom, not to raze the whole kingdom but to bring it firmly under her rule for grander, larger conquests in the future.  Due to the size of the kingdom, she made a deal with Demogorgon for basically manpower support.  So there are the dragons direct armies and a cult of Demogorgon with its cultists and demons.  In the beginning of session 1, the players witnessed a key event they still haven't learned the importance of.  Silver, an ancient silver dragon and self-proclaimed protector of the kingdom, attended a seaside celebration in his honor.  A group of storm giants marched out of the sea carrying a large black chest.  They have been allies with the kingdom and Silver for a very long time, so Silver was at ease and delighted by their attendance.  Once they laid the chest at Silver's feet, an obvious patronage to the dragon on his celebration day, Silver opened it expecting a kind patronage.  Instead, dozens of magical black chains shot out of the chest, far more than could logically fit in the chest, and bound the dragon in seconds.  Ignoring the hundreds of on-lookers in attendance, the storm giants immediately dragged Silver into the sea and disappeared before anyone could get their wits about them enough to respond.  That began the siege on the kingdom. effectively step 1 of the ancient red dragon's plan.  I took Ilmryth from SKT; an adult blue dragon that can shape-shift into a storm giant.  She managed to imbed herself in Maelstrom (just like SKT) and her main job is to ensure Silver's imprisonment in Maelstrom.  The red dragon didn't want to kill Silver immediately...she wants him to watch his realm fall then plans on imprisoning him via the Imprisonment spell in a locket around her neck.  To get the storm giants to comply, Ilmryth secretly planned the kidnapping of the Queen, who is being held captive high up in the mountains that border the sea by a clan of cyclops who revere the ancient red dragon as the foretold coming armageddon.
That's basically the 30,000 foot view of the story.  The two different factions, the red dragons orc/giant/etc army and the demons and cultists, aren't connected at first to the party's view.  Not until the city they are in gets attacked at the exact same time from the inside (cultists and demons via portals to the Abyss in the sewers) and outside (large orc armies with dragonborn leaders and giants, etc).  They find evidence connecting the two.  Their backstories are filling in a lot of the side quests but its mainly focused on them trying to figure out what is happening, deal with both sides of the opposition where they pop up, etc.

Obviously neither an ancient red dragon nor Demogorgon can be trusted and their contract is tenuous at best.  Both sides are plotting against the other at the right time...the players could use this to their advantage if they figure out how.  I'm really leaving the possibility open to them; they could fight one or both of them, together or apart (together would be crazy because it'd be 1v1v1, not 1v2).  4 of the 5 players came up with really good backstories that will help get them prepared for those last fights; an arch-hag, frost giants (basically took the chapter from SKT of Svardborg), a location that is my version of Skullport where the players father is enslaved by a Rakshasa, and an artificer who is chasing a myth called the Grand Alchemist...supposedly an artificer who might have achieved immortality or something.  He has little workshops spread throughout the realm with hints and schematics for the artificer to find.