A swift moving invading army (from another plane) composed of 6-10 HD undead * and all of the evil humanoid races has conquered Tethyr and is swiftly beating back the local amn/tethyr army. In just 3 months time they are partly into amn and calimshim and a strike team of high HD skeletons is nearing the mountains of the lake of steam. The invaders are +8 LA humanoids who are masters of planar travel. They are stranded on Torril ** and needed to annex Tethyr because it is the perfect spot to create a permanent gate to their home plane. Once opened, their god-king CR 30ish + divine levels will come through to torril and assuming the plan sticks, kill off Lathandar. (The gates are sun powered, I guess.) Until that happens, the invaders need native clerics to gain access to divine magic. Did I mention the twelve old or ancient dragons they are controlling with an artifact in order to move troops long distances quickly?
Our 14th level party of 6 is based in Scornubel and have been running around the realms looking for the armor of heron (found the sword of heron last game arc near the lake of steam). These items are rumored to be able to defeat the prophesized invaders somehow. Only the party cleric-7/fighter-7 of Lathandar knows how though.
Next some uppity demons in the unapproachable east will divert our attention. After that we travel off plane to gain some insight into the enemy. (We've only met one of them and he soundly defeated a group of 14 PCs by himself after a slight amount of softening up.) Seems there's this other plane where the invaders once lived. Sucked it dry of most of its resources and left.
* The cool thing about the undead is that in last 2e game we played 4 years ago, the party was investigating a gate where vast numbers of undead were being called to. Every graveyard within a hundred miles of Silverymoon was spitting up skeletons and zombies and they would wander southwest to the a large staging area. Last we saw, there were thousands of these things standing around and a suspicious set of arcane columns. The columns were about to open as a portal and the undead were going to step through. We never did get back to that. The DMs cannot decide if those adventurers will show up to the final showdowns of the campaign as it turns Epic.
** They first conquered the lost continent opposite Faerun whose name escapes me completely. By the FRCS, Amn trades with this continent but not in our game. It's still basically unknown. This is where the above undead were going. There they were enhanced, armed and eventually portaled by to Tethyr.
I say we and our a lot up there. That's because I both play and DM in the campaign. There are three DMs in the group and we take turns with the stories which is why I call the sword of heron the last story arc. I DMed the sound thrashing and will DM the off plane interlude.
Oh, and no, we don't ever follow what is happening in the novels.