The Mortal Cult Of Blood Knight Gaius is a low-level adventure designed for use with Knave or B/X (although it feels very easy to adapt to other osr.)
This is not the first thing you'll notice about it, however.
The first thing you'll notice is the striking art and graphic design.
Blood Knight is stark and grimy, and I basically felt ambient synths start playing in my head when I opened it up.
The PDF is 20 pages, and the adventure within orbits around its central character: the vampire Gaius. Gaius has a *strong* gothic vibe, being torn between his noble goal and his sanguine desires, and in general the adventure presents the vampires are tragic monsters---with emphasis on both the tragic and the monster.
That said, the module's setup is a fairly hard subversion of all other vampire modules I've seen. The PCs know going in that Gaius and his cult are vampires, but the vampires aren't automatically dangerous to the PCs. Their whole deal is about denying their urges and only killing with permission, and they're genuinely good hosts if all you want out of the adventure is an eerie, tense place for the PCs to rest in perfect safety.
If the party wants to treat the dungeon like a regular dungeon, they absolutely can and the module is formatted expecting this, but there's a fair amount of attention in the room descriptions paid to the way the vampires' cult works on a day to day basis. If you go full violence from the jump, it'll feel a bit like you're dungeon-crawling someone's home---and that alone would sell me on it, but there's more.
The module includes an excellent dungeon map, unique spells, interesting NPCs that the players nevertheless aren't forced to interact with, multiple ending conditions, and the text sticks its gothic tone right up until the last two pages when it gets slightly cheeky.
Overall, I think this is one of my favorite osr adventures. It's easy to drop into a campaign, it's well-written and atmospheric, the players set the pace through it, and the players decide which aspects of it to engage with. Even without the impeccable art and layout, it would still be versatile and easy to use, but as a complete package it is a huge value-add to your old-school adventure library. I strongly recommend getting it.
Minor Issues:
-Page 10, First Disciple Matsunaga's entry seems to be cut off here.