Please help me finding adventures which would fit this campaign as additional adventures. I am also looking for ones which could fit setting and story and bring the party above level 15.
I can think of a particular Underdark adventure wherein you raid an evil aboleth city to put an end to an evil magic stemming from there. It was like in Dungeon #144. The authors were kind of hacks, though...
"Home Under the Range" in Dungeon #134 might work for you too if you want the PCs to do even more Darklands travel.
Final Resting Place is a decent underdark themed adventure, I think it's a Michael Kortes adventure. Not sure of when it appeared, I think it was in an issue between AoW and ST.
Edit: Yay for the search function: it's in issue 122.
Edit 2: And, it looks like it's between SC and AoW.
Edit 3: Did Michael Kortes run an Underdark campaign or something? I just realized Home Under the Range is his too.
If you're willing to put in some muscle, you might also consider Kingdom of the Ghouls as a way to continue the adventure past the end of Second Darkness. It's a 2e adventure, but it's a classic.
I was looking through some published adventures searching for some ideas to fill the gap. I found a good one that fits nicely with the feel of Second Darkness. It is written for sixth level and has a drow bad guy and an alien bad guy.
Its chapter three from Goodman Games' Dungeon Interludes, Dungeon Crawl Classics #14. Its titled The Hunter of Worlds.
A short synopsis:
Spoiler:
A drow assassin tries to poison the PCs. They chase after him and he flees into what turns out to be the lair of an alien Xill hunter, who's doing weird xill-kind-of-stuff.