Y is for YS1 The Outpost of the Outer Ones.
There are no classic adventures that have a Y or a Y-related code.
Thankfully there is an adventure that does have a Y code, YS1, and it is set up very much like the classic adventures. Created for OSRIC it can be played using AD&D 1st Edition.
YS1 The Outpost of the Outer Ones was written by Jeremy Reaban. I have featured some of his products here in the past.
Y in this case might stand for Yuggoth, which is the home-world of the Mi-Go, or at least one of their outposts. This adventure, designed for characters 6th to 10th level for any old-school game, heavily features the Mi-Go. While he describes it as a "Science fiction" "dungeon crawl" only a tiny bit of work is needed to make this one horror or a mystery. Afterall, people are going missing, strangers are showing up in town and there is that whole eerie cave system.
Like most of the old-school adventures, this one is light on plot and heavy on the dungeon crawl atmosphere, and that is by design really. The adventure is simple enough but there is so much more that can be done with it if you want. Note: I should point out this is NOT a criticism of the adventure, quite the opposite really.
So basically the Mi-Go are in town and they are doing what the Mi-Go do, removing brains from bodies and putting them into other bodies or their special cylinders. The brains stay alive and are even immortal after a fashion. They are also experimenting on the local fauna. A couple of things in this adventure jumped out as me as hitting that 70's/80's nostalgia sweet spot. There is a Flumph the Mi-go can't figure out. A bionic Sasquatch! (I mean really, was this written just for me?) I biologic towel, a Valley Girl brain, and this whole "Escape to Witch Moutain" vibe about it. There is a witch and Swanmay in it as well.
Personally I would take Jeremy's advice and expand the module a bit. Have the party meet the old witch Gwen in her "old" form, but then encounter her again when she is in one of the brain jars and then again when she is in her new body. Also, I'd make all the Mi-Go's human form all look roughly the same; perfect, blonde, blue eyes, devoid of any real personality. Like something out of Village of the Damned. Liked they learned how to be human by reading it in a book.
I'd also make their plans a little more nefarious. This is a scout group looking to colonize this planet. Makes that bionic Bigfoot look a little more scary if you ask me!
Obviously, a good companion to this adventure would be Jeremy's own OSR Warlock. Make Gwen a warlock AND the one responsible for bringing the Mi-Go here. I'd also play it under Astonishing Swordsmen & Sorcerers of Hyperborea. Give it that "colder and darker" feel that AS&SH can provide. Plus there are already a number of good Lovecraft Mythos beasties in that game.
My biggest issue with this adventure is where do I use it? I have so many choices to be honest. I could easily slot it in as a "Monster of the Week" story, but that would sell it's potential short. I could make it part of a larger campaign, but I would also want the Mi-Go to be more that just a one shot.
In any case I know this will be a fun one.