The Swamp Daughters of Marshsund is an adventure by Oliver Korpilla and Mattia Giardini. Arct is by Bernd Jans (cover), MonkeyBlood Design (some elements used in map design), and the public domain. Cartography is by Dyson Logos and Oliver Korpilla. The publisher is Mount Parnassus Games. No level is given in the text, but this interview at the Goodman Games site indicates that it is a level 3 adventure, as does the product page on DriveThru RPG (“level 3 or higher”).
I am going to suggest that the seeds for this adventure could be set even in the 0-level funnel, as it provides a mini-setting with a hex crawl, rules for foraging, and local communities as well as the titular problem driving the narrative. Is there a dungeon? Yes, but it is not terribly large, and the setting material can drive play for many sessions before and after the Swamp Daughters themselves are resolved. This adventure is a bit of an odd bird with published DCC work, because there is no doubt that it is pitched towards campaign play rather than use as a one-shot adventure. The generous OGL terms all but invite you to expand upon it.
Although The Swamp Daughters of Marshsund came out in October of 2014, as of this writing I haven’t seen a lot of discussion about it, so I am going to avoid spoilers. As with The Headless Horseman, it is clear that the authors’ experience with other games informed their design here. In fact, the words “Stat Block (DCC)” appear before every statblock, suggesting that a version for a different system was at least considered.
If you are looking for a plausible basis to build a campaign setting, this provides a simply developed but narratively complex starting point. The actual adventure could be built up to, so that the players felt that they actually know the Marshsund villagers before events unfold. In this case, the judge would be well advised to create some additional marsh encounters, and place other potential adventures in the vicinity. The Croaking Fane would definitely fit in here. The Folk of Osmon is more of a stretch, but is a possibility.
The people of Marshsund have allied with strange powers, but for almost a hundred years nobody bothered investigating. Now the daughter of a powerful lord has gone missing in the Dystermarsh, and he is looking to you for help. Can your players unravel the mystery of the Swamp Daughters and find a solution in time?