Main Story

Act 1 The Sea of Fallen Stars:
Set in/around the Sea of Fallen Stars.
The party gets abducted and sent into a recruitment camp/program of The Cult of Azothia. They escape, and discover a town that has been destroyed and corrupted by the cult. The party is introduced to the politics of the city states that surround the Sea of Fallen Stars and discover that everyone is on the brink of war.
Any personal quests set in/near the Sea of Fallen Stars would happen here in Act 1.
The party is hunted periodically by Cultists and discover some of the lieutenants of The Cult that are leading the charge. Most likely Mazus Crane and Mara Crane. Some of these hunts involve the constantly moving flotilla that the Crane twins control.
The party will have multiple run-ins with Hastur /his cultists, and will discover what Hastur’s ultimate goal is. This would climax with a battle with one of Hastur's lieutenants (either Mazus, or Mara), where we find out that Hastur claims to have someone from Kimlen Barrowfield's bloodline and is headed deep into the Underdark to open the gate toThe Far Realm .
Finding the entrance to the Underdark is a difficult task, as most people who are unlucky enough to find an entrance rarely make it back to the surface to tell the tale. The group eventually runs into an older Cleric named Alessi Woodbracer , who after revealing his harrowing history, is able to direct them to a new opening to the Underdark. An entrance hidden deep in the forest of Alessi’s old friend, the Druid, Oloro.
Act 2 Venturing into the Underdark:
The Party chases Hastur down through the levels of the Underdark culminating in a standoff at which time Kimlen is tricked into opening Marwin's Door. We encounter a terrifying creature that has been waiting on the other side of the Marwin's Door in the Far Realm, who stalks us as we enter the Far Realm. We eventually encounter the creature, and are quickly overpowered. Facing certain death for us, and catastrophe for the Underdark & our homes from the creature if it is able to enter our world, Vardan uses the bag of holding maneuver, and we are cast into the Astral Sea.
Personal quests related to the Underdark would happen here.
Act 3: Lost in the Astral Sea:
The party has to find a way to get back and stop this creature from terraforming the planet into its home terrain. We traverse the Astral Sea tracking a color pool that can act as a portal to send us home to stop the creature.
Act 2 notes:
Hastur knows where Marwin's Door to the Far Realm is, but cannot open the door. For that he needs Kimlen there to perform the ritual. Obviously Kimlen wouldn't go willingly, so Hastur claims to have someone else from Kimlen’s bloodline - his mother Tali Barrowfield. The veil between worlds is thin the deeper you go in the Lowerdark, and it would be the best place to open a door to the Far Realm. Hastur knows this.
Marwin’s door is very deep in the world, and very difficult to get to. Located deep in the lowest section of the Underdark, 20 miles below the surface, and on the other side of the The Glimmersea
Hastur tried to find this place before, with the help of his cultists. But there are Aboleths deep in the Glimmersea. The first time he tried to cross the Glimmersea, he and his crew were attacked, but he managed to make a deal with one of the larger, and older Aboleths. He would help this Aboleth by feeding it humanoid slaves for whatever dark purposes it desired. In return, he wanted safe passage, and knowledge of the ritual Marwin did to open the door. Hastur has not yet been able to deliver on his promise to the Aboleth, and he is hoping that he can sacrifice his cultists that he brought with him to the Underdark, as well as some of our party to appease the Aboleth.
Aboleths get larger with age, and do not die of old age, so they can theoretically live forever (unless they are killed). They also have perfect genetic memory. This means they can relive any moment in their history with perfect recollection, and can even perfectly recall their ancestors' memories as well. If Hastur made a deal with an Aboleth to gain it's perfect recall memory of what Marwin did, that would be extremely useful.
The party could venture into some of the cities run by various factions of Drow & Deep Gnomes so that we aren't constantly fighting for survival, but the paths between each city would get more and more difficult the deeper we go. Maybe in the last city before we reach the Glimmersea, we come across a small town where the people inside all seem off in some way. Maybe their eyes don't seem to be able to focus on you when you speak to them, or their eyes seem to randomly go in 2 different directions. Then we come to find out that they are being controlled by this Aboleth to do it's bidding. Some of them are even cultists that thought Hastur was bringing them deep underground for a powerful ritual, but really he was leading them like lambs to the slaughter to the Aboleth to be enslaved.
At some point the party stumbles upon a group of Githyanki in the Underdark who have formed a hunting party to seek out and eradicate a colony of Illithids (Mind Flayers). This hunting party is part of a rite of passage for the Gith, and considered a great honor. The party could either help save them as they are getting overwhelmed by Illithids, or the Gith could stumble upon the party fighting something else, be impressed, and then invite the party on the hunt.

Structure

Exposition

Magic is Waning: Arcane & natural magic has been fading in the world for the past couple centuries and nobody knows why. Rumors & theories run rampant and have put many people on edge with paranoia, a sense of existential dread has begun to sweep over many people. Divine magic, pact magic, and abhorrent magic seem to be unaffected. Even Dragons are beginning to fade since they are inherently magic. Only the biggest, and strongest dragons remain.
On the Brink of War: Aris Mournen “The Mourning King” has conquered much of the eastern lands to control what some think is a well of magic. It is unclear what his intentions are but he is fiercely holding the lands around it, and with magic fading from the land this area has become extremely desirable & valuable. This is causing civil unrest in neighboring city states as they feel their grip on power slipping.
New Religions: New religions are beginning to emerge, mostly to try to explain why magic is fading, or more accurately, to cope with this revelation. The Cult of Azothia is using this as an opportunity to sow chaos and paranoia, feeding the growing possibility of war.
Elder Gods: All of the pantheons of gods know of The Far Realm, but only a few know actual information about it. They mostly know that The Far Realm and the elder gods that reside there are older than the all the other planes. Some think it might be a remnant of a previous universe that somehow managed to live beyond it's collapse. Some even say The Far Realm is its own separate universe and is starting to encroach on our own. Most of the gods stay away from The Far Realm and its inhabitants as there are rumors of some gods being consumed by it.

Conflict

The Kingdoms that surround the Sea of Fallen Stars are on the brink of war. The Northern Kingdoms (Thay) VS the Southern Kingdoms Alliance (Aris)