The Dales? Cool; stacks of potential there. However, I would personally drop the Loviatan theme except as it relates to Tears for Twilight Hollow. There are enough evil powers already at work in the Dale.
For an overall goal, I would be looking at the party sundering the alliance between the Zhentarim and the drow that infect Cormanthor. Here's a suggested outline:
Adventure One: Galath's Roost
In the Zhentarim citadel, make sure that the party finds some Zhentarim war plans for the Dales and perhaps details of the alliances with the drow. Basically, things important enough to convince them to return to Mistledale ASAP.
Adjust it for your party's level and then don't let the portal work to return them to Mistledale so that, as they wander through the Thunder Peaks (throw in some ogres and orcs), they find:
Adventure Two: Tears for Twilight Hollow
You may need to adjust some of the stats to reflect a slightly lower party level (although that also depends on whether you treat the journey through the Thunder Peaks as an adventure in itself).
Perhaps make the Loviatans more entrenched than in the existing adventure, and then include a Banite Zhentarim emissary (a skymage if the party is powerful enough or even a Banite blackguard, otherwise a Banite cleric) seeking an alliance with the Loviatans against the Dales.
Adventure Three: Return to Mistledale
Journey through the Thunder Peaks culminating in a battle with the red dragon, Thraxata, who lairs in the Bloodhorn.
Adventure Four: Mistledale Under Attack
Returning with news of the planned or existing alliance between the Zhentarim and the drow, the party return to find Mistledale under attack. The adventure will thus consist of multiple skirmishes with Zhentarim and drow raiders, plus a dramatic rescue of a farming family etc..., hopefully to build some attachments to Ashabenford (or whatever other village/ town/ city you have chosen).
Adventure Five: In Search of the Drow
Throw in the free adventure, Fang, Beak & Claw from the WotC website for a slight change of pace and then have the party off on a drow pogrom. Let them find, a la the set-up in Lords of Darkness, a drow surface lair which leads by portal to an Underdark lair.
The Underdark lair may have been wiped out by the Scarlet Tide just in case you really want to use that Dungeon 85 adventure. There is an underground lake beneath the Dales (the Lake of Shadows, IIRC) so it still fits with FR canon.
Adventure Six: The Scarlet Tide
Actually, I don't have this edition of Dungeon so no comments here, other than include something to link the PCs back to the main adventure. Perhaps they find a Zhentarim corpse that has a map showing the location of the main Zhentarim outpost.
Adventure Seven: Against the Drow and/or Zhentarim
From here on in, the party takes the fight to the big bad guys. Cradle of Madness could be an ancient temple of Moander located beneath the Dales and being used by the drow or Zhentarim as a base... except the lingering evil is slowly having an insidious maddening effect.
Ideally, the map found in Adventure Six should lead the party here. At this point, your campaign has reached a natural conclusion and the, where to next?, question will really depend on what you and your players found to be most interesting and exciting along the way.
Maybe at this point a new threat arises in the form of Loviatar-worshippers, alternatively you might be now in a good position to introduce City of the Spider Queen as an almost natural extension of this campaign? If you choose the latter course, make use of extensive foreshadowing in the Underdark and drow encounters to show how things are changing in the Underdark thanks to Lolth's silence. Reading CotSQ will probably give you more ideas.
HTH.
Cheers
NPP
