Good adventure or adventure series featuring slavers as the villain

[Gilladian]

What about Night Below?

I was about to mention that one, too. It's a whole campaign in a box, and it does have its boring moments (fighting the trolls, a lot of the side-adventures in the middle book), but a good DM can make a great campaign out of it. It focuses on one person who vanishes, taken captive by the slavers, and the PCs are supposedly on an epic rescue mission through the whole campaign, although it becomes a little unrealistic as time passes.

The villains start out as orcs or goblins, and eventually end up being kuo-toa and worse...

 

[NewJeffCT]

I was about to mention that one, too. It's a whole campaign in a box, and it does have its boring moments (fighting the trolls, a lot of the side-adventures in the middle book), but a good DM can make a great campaign out of it. It focuses on one person who vanishes, taken captive by the slavers, and the PCs are supposedly on an epic rescue mission through the whole campaign, although it becomes a little unrealistic as time passes.

The villains start out as orcs or goblins, and eventually end up being kuo-toa and worse...

Interesting - 5+ years ago, I started off an epic campaign with much the same idea as Night Below, it seems, even though I'd never played it, DM'd it or even read a plot summary of it. The players were searching for a missing woman (a young priestess of the God of Freedom & Liberty, but also a twin) who was captured by followers of the God of Tyranny & Slavery. Because she was a twin, the slavery guys thought she was the prophesied twin who would be impregnated by the Tyranny & Slavery god's high priest, and the child's birth would allow them to activate a powerful artifact to help them enslave the world (i.e., an Ark of the Covenant type artifact). So, the players had a 9 month timeline and started off fighting goblins and orcs, then the bad guys got progressively worse, until they had the showdown with the high priest and hit pit fiend bodyguard...