Pumpkin Jack. It’s a 3D platformer. I haven’t played it in a couple years, but I remember it being mostly linear. Not a ton of collectables, but some. 11 months out of the year it’s a pretty “meh” game, but it absolutely NAILS the Halloween aesthetic. Not “horror” or “scary” or “autumn” but very specifically Halloween.

MediEvil is similar, though much older. I have only played the original for PS1, though there is a modern remake on all platforms that looks pretty good. Not quite as explicitly Halloween-y, but still pretty close. Flawed in its own ways, but I would still say a better game overall than Pumpkin Jack. The levels were a bit less linear and it was a bit more like an adventure game than a platformer.

Luigi’s Mansion is a classic too.

A lot of other games have levels or worlds that are good for Halloween even if the whole game isn’t. Like Pumpkin Hill in Sonic Adventure 2, or Subcon Forest from A Hat In Time. Honestly one day I want to compile a list of all of these themes areas across my favorite games and the play all of these levels seasonally.

For spooks, Phasmophobia is a good one. They do a Halloween event every year too 🎃👻

The 7th Guest if you want to play a puzzle game with campy acting

also Deus Ex Randomizer has a Halloween mode

The secret of Monkey island games have a tropical Halloween feel with ghost pirates and root beer

Kentucky route zero for a slower ghost story, dear Esther for a walking sim ghost story both with great soundtracks

I remember they used to do gm run player events around Halloween in EverQuest back in the day and on project 1999, not sure if any mmo or online games theses days still have human gms

How about Jack in the dark, the little game that shipped with the cd version of the two first Alone in the dark games?

Eternal Darkness for Gamecube.

I can’t invest the time in MMOs anymore but I always miss them around this time of year. Guild Wars/Guild Wars 2 did a great job of it.

Costume Quest is great for a single player game.

I’ll also throw out a classic - Ghost Master. You manage a team of ghosts haunting different locations.

You already mentioned the two games I know of that do halloween events, so here’s just a list of spooky horror games that are extra fun to play in October:

I liked Indigo Park. But, like… Not because the gameplay. I just really like Rambley.

But Resident Evil (any other than Outbreak) is spooky.

Amnesia, Outlast, the old Hugo point and click adventure games for DOS, Phantasmagoria, Phasmophobia, Slaughterhouse, Castlevania…

If you’d like a less serious, funny comedy-horror game, I enjoyed The Upturned.

Some less silly games I’d also reccomend are 10 Dead Doves
and
Withering Rooms

Killing floor used to do good holiday events, but it’s been years since I’ve participated, they may not anymore!

Not particularly a whole game but the halloween day/nite in bully. It always gave me the perfect Halloween vibes.

The Cursed Forest is fun.

Vampire Savior

Sounds like you’re focused on multiplayer games, but the Echoes of the Eye expansion pack for Outer Wilds is a phonemonal “spooky” and eerie game without being a true horror game. Highly recommend both it and the base game.

Look Outside just had a major 2.0 patch. Solid horror game. And it’s 30% off at the moment.

Steam link