One of the more amusing things about OSR gaming is that seemingly there are more new rules sets than adventure modules. Thankfully, a few people are making adventure modules, some for specific gaming systems, but others, like Liberation of the Demon Slayer from Venger As’Nas Satanis (I'm guessing that's not his real name) are simply written with vague old school stats.
Okay, perhaps that is not completely true, while Liberation of the Demon Slayer is mostly a dungeon adventure (consisting of 6 levels), it starts off with about a dozen pages of various house rules, tips and suggestions for old school style games. Some of these are interesting, like a variant of the ability check that uses a different number of d6s according to the difficulty. You roll under the ability score on 2d6 if it's easy, 3d6 if it's average, and 4d6 for a difficult task. (Actually I think Traveller 4 and 5 do something like that, but I haven't seen it in a D&D based game).
On the other hand, I didn't really like the suggested rule for magic, requiring the player to make a special roll every time they cast a spell. For one, it simply adds another roll to the game, for another, on a 1, the spell backfires, affecting the caster instead of the target. Which in the case of a spell like fireball, would mean the end of the party, not just the caster. Some people like games that capricious, but in my experience, that sort of thing works best in one shots, as it's going to happen eventually.
Other rules include alternate means of advancement (gaining a level in a number of sessions equal to that level), "exploding" damage (when you roll the maximum on the die, you roll again. Spelljammer used this for gun damage), and even a table for dark secrets the adventurers might have.
The Dungeon Itself (Mild Spoilers)
Unlike a lot of large dungeons (6 levels, not quite a megadungeon, but close)it does have something of a hook. The PCs must venture forth into it to retrieve a magic sword, the titular demon slayer to help fend off an invading demonic horde. You'd expect it to be at the bottom of the dungeon, or at least close to it. The opposite, rather.
One level features a crashed spaceship. This is getting to be something of a cliche, but it's handled pretty well here. There's a level with surprisingly friendly drow. The last couple levels are devoted to a thorougly evil temple, and here it's probably the most lurid, full of sacrifices and such.
The catch with the dungeon is that it's not particularly granular. The 1st level is more or less for first level characters, the 2nd and 3rd mostly combat free and suitable for low level parties, but the 4th and 5th levels are probably more for 6th level characters, and the last could be a challenge for a 10th level party.
Even if you use the suggested leveling scheme (based on the number of sessions played, rather than gold for XP), I don't see how PCs would be high enough level to venture to the 4th/5th levels. Even in the intro, the author says he expects it to last 7-13 adventuring sessions, which using his plan, would leave PCs at 3rd or 4th level.
Heck, I'm not even sure how the PCs get past the first level. The second level is flooded, but beyond that, there doesn't seem to be any stairs down from the first level.
Perhaps one of the downsides to using such a vague ruleset is that a lot of things aren't detailed. For instance, there are a number of friendly NPCs. What are their classes, their levels, their stats? Some get a minor bit of detail, others get none. Similarly, for inspiring the name of the adventure, the demon slayer itself is rather dull when it comes to magical powers. It's basically just a +1 sword, +3 vs infernal and fey creatures. Oh sure it's intelligent, but the usual sort you find in a magic sword (vain and pushy).
It's actually somehwat unclear how the PC with the sword is supposed to fend off a demonic horde. A 1st or 2nd level character with even a +3 sword is little match for a 7 HD demon (which I think the invading ones are, they are given stats on a lower dungeon level), much less horde.
What is also unclear are some of the stats for the monsters, things don't always make sense. For instance, there is a medusa with 13 hit dice, but an attack bonus of +5. Damage per attack can also be odd. For instance, there is a half-orc (no class given, though he's described as a fighter, rogue, thief, and assassin) does 5d4+3 with a +1 dagger or 1d6+4d4 with a crossbow. Where does that 4d4 bonus come from? Is that supposed to be sneak attack damage?
Like perhaps many people my age, I tend to think the modern day era kind of, well, sucks. Part of this is no doubt simply just nostalgia, but I do think there is something of an element of blandness the permeates things today, from books and movies to music and games. Sort of like how the 1950s looked to someone who grew up in the Roaring '20s. While it seems born largely out of a desire to not offend rather than prudery, the end result is the same.
In that regard, Liberation of the Demon Slayer is a blast from the past, like the art on the side of custom van turned into a D&D module. Tacky and lurid, but in an awesome way. The old fantasy magazine Heavy Metal often gets invoked for products like this, but that magazine had a lot of different sides to it, featuring a lot of pretentious, bewildering artistic stuff along with the outlandish and sleazy.
On the other hand, the dungeon design itself is a little rough. Some things don't make much sense, even by dungeon standards, and there's not really enough in the easier levels to get characters high enough level to do the lower levels. So the idea of this product is perhaps a lot better than its actual execution.
Probably the best use of it is to divide it up into 3 different dungeons, or perhaps simply use the room descriptions to stock your own dungeon. Many encounters are inventive and the author is always enthusiastic.