Galgenbeck Sacrifice is is an adventure for Mork Borg that adds a lot more flesh to its titular region.
The PDF is 67 pages, with a layout style that perfectly matches the core book. The art is vibrant, splashy, detailed, and evocative. The layout breaks conventions in intentional ways, and pages that are dense with information are contrasted against pages with a lot of whitespace, giving the reader room to breathe.
Sacrifice's writing also perfectly mirrors the core book. It's punchy and short, but dense with information. Some information in the core book is recontextualized by Sacrifice (mostly concerning Josilfa Migol,) but the feeling is far more of a sequel than of a fan work.
Contents-wise, there's information about the atmosphere in Galgenbeck, local places to visit, rumors, encounters, and some fun NPCs, but the meat of the book is the adventure. It starts with a compelling hook, has a recurring superboss, and explores an interesting premise---an avatar of Nechrubel is harvesting citizens, erasing them from living memory.
It's somewhat easy for players to fall off of the intended track of the adventure, and the GM might need to do some creative freewheeling if they fall off but keep pursuing it, however in general the atmosphere is strong, the stakes are high, and there's a great sense of pacing. This is an adventure that matters for the setting, and will fit very well with groups that like a gloomy but still heroic tone.
Overall, Galgenbeck Sacrifice feels less like a standalone supplement and more like an extension of the core book. It's packed with content. It interacts with the central plot. It does a stellar job of bringing Galgenbeck and its strange inhabitants to life. If you like Mork Borg core, it feels safe to call this a must buy.