Taken!

This is the first stage in a series of connected encounters. For that, it requires more of a preface. This was an idea I had in writing my original story idea down, and I think it would be fun to incorporate into an actual adventure. Basically, the party is attacked by a gang of small creatures - goblins, or the like, overwhelmed and "killed", awaking as prisoners in the darkness. At this point, they must find each other, their belongings, and the way out.
Taking them prisoner is the first step, and likely will need to feature an almost endless surge of monsters. Once they are prisoners, there is less interest in battling their way out, its more about the uncertainty of where they are and what is happening, leading them to improvise in order to recover their spoils and escape.
For this adventure, it would probably fit better to use kobolds, but that might be a stretch. On the other hand, there would be a glorious element of humor to overrun a group of mid-level adventurers with a band of kobolds! To make that work, I'd literally swamp the party in kobolds. Send them in, let them get massacred, and just keep pouring more in until the party is overrun. The key is that the party isn't actually getting killed, they're just getting zeroed out so they're unconscious.
Adding a generous portion of winged kobolds would help, and there is no harm in throwing in a few larger types, like a bugbear or hobgoblin or something. Frankly, the monster you use is irrelevant. I thought of using Drow in another adventure for similar effect. It just fits with the idea of the cultists and the kobolds to use them. As for motive, they could have tracked them here, amassing a large number of allies to grab the party, with plans to deliver them to the higher orders of the cult.
The party's challenge is to find a way to escape the warren of the kobolds, with their belongings, before they can turn them over to the other faction.