Dont you think the initiate would help redundancy ?
How effective is the loyal deep one assault in 4 colour deck ?
How essential is Claude Owen that you need to include him in 2 copies buth no other Yog cards
If yo run infinite Marcus with Shub why not include under the porch?

Initiate is restricted, and so I could only pick him or Jamburg. I tried a deck with Initiate and he just didn't guarantee the win fast enough. Jamburg is much more versatile in my experience.

Deep One Assault is there to kill things like Prepared Alienist, Flux Stabilizer, or Nathaniel Peaslee. I don't need a huge domain for it.

I chose Claude Owen because I very much wanted to be able to play characters from my discard so that I could get maximum use out of things like Grasping Chthonian, recover more quickly from a Dimensional Rift, or retrieve a crucial Jamburg. The other card I could have chosen would be Corrupted Midwife, which would of course be in faction. The reason I chose Owen is because I don't have to pay for him when the combo is up and running (and it is by turn three almost always), he gets me two characters for three cost (anyone I would want to return from my discard would be three cost so I'd have to pay at least that much for one character with Corrupted Midwife), and I can use his ability on my opponents turn, which helps with instant support destruction.

Under the Porch only fetches Shub characters. I have the Festival (+either Claude or the Necronomicon--this deck always has whatever support card it needs thanks to Underwater Vault)/Shocking transformation to tutor out Grasping Cthonian, which is the only Shub card worth searching for.

I'm also considering cards like Deep One Rising and the Large Man, but I need more playtesting. Some important things to think about when playing is to save your support destruction for supports that break your combo, to always leave Jamburg in your hand where he's safe (I've never played against Hastur discard so I'm not sure how that would go), and to always have an Underwater Vault (or two!) in play so you can get out a Plague Stone in case they have something obnoxious like a Professer Herrmann ready to snag your Jamburg being protected by Decrepit Wizard. This deck recovers very quickly from board wipe effects because everything is free, Jamburg and Uroborus will always survive, and you can go get yourself a Shub Necronomicon at any point.

Resourcing early-game is obnoxious but manageable. Festival means you don't have to worry to much about it.

You use sacrifice Uroborus to activate Ritual of Summoning. Uroborus returns to play insane. You sacrifice him again, and so on and so forth until you have infinite success tokens on the Ritual. Now you can play any character at any point for free. Flooded Vault, the Festival, and Shocking Transformation all speed this along. I've chosen to have my use of the combo revolve around Marcus Jamburg because a) he protects the Ritual, infinite Ice Shaft and infinite Flooded Vault (instant access to solution-y supports like Necronomicon, Plague Stone, and Book of Iod).

I've been considering adding in the day version of Plague Stone (the name's escaping me, the one that lets you draw cards) and swapping Twilight Cannibal for Ghoulish predator

Initiate is restricted, and so I could only pick him or Jamburg. I tried a deck with Initiate and he just didn't guarantee the win fast enough. Jamburg is much more versatile in my experience.

Deep One Assault is there to kill things like Prepared Alienist, Flux Stabilizer, or Nathaniel Peaslee. I don't need a huge domain for it.

I chose Claude Owen because I very much wanted to be able to play characters from my discard so that I could get maximum use out of things like Grasping Chthonian, recover more quickly from a Dimensional Rift, or retrieve a crucial Jamburg. The other card I could have chosen would be Corrupted Midwife, which would of course be in faction. The reason I chose Owen is because I don't have to pay for him when the combo is up and running (and it is by turn three almost always), he gets me two characters for three cost (anyone I would want to return from my discard would be three cost so I'd have to pay at least that much for one character with Corrupted Midwife), and I can use his ability on my opponents turn, which helps with instant support destruction.

Under the Porch only fetches Shub characters. I have the Festival (+either Claude or the Necronomicon--this deck always has whatever support card it needs thanks to Underwater Vault)/Shocking transformation to tutor out Grasping Cthonian, which is the only Shub card worth searching for.

I'm also considering cards like Deep One Rising and the Large Man, but I need more playtesting. Some important things to think about when playing is to save your support destruction for supports that break your combo, to always leave Jamburg in your hand where he's safe (I've never played against Hastur discard so I'm not sure how that would go), and to always have an Underwater Vault (or two!) in play so you can get out a Plague Stone in case they have something obnoxious like a Professer Herrmann ready to snag your Jamburg being protected by Decrepit Wizard. This deck recovers very quickly from board wipe effects because everything is free, Jamburg and Uroborus will always survive, and you can go get yourself a Shub Necronomicon at any point.

Resourcing early-game is obnoxious but manageable. Festival means you don't have to worry to much about it.

Thanks for your insight

Now that I've thought more about it, I think I was overly harsh on Beneath the Porch--I think it probably would work pretty well in a deck like this, and it would allow me to use Grasping Cthonian's ability twice per Cthonian without having to revive it from the discard. The downside is that it's a location, so it couldn't be searched for with Flooded Vault. Hopefully I get around to playtesting it

I really like the deck idea, but I feel the combo is too slow and too fragile, expecially since you have few characters. How long does it typically take you to win a game?
I suppose I'd have to playtest it myself. It's difficult to get a good feeling how well it works by just looking at the list.

So the four or five times I playtested it I got it up almost always on turn 3, and once on turn 4. Playing against rush decks I sometimes will be one or two success tokens away from losing, but when this deck hits, it hits hard. I'm gonna take out all of their characters, and because I can play Jamburg on their turn as well as mine they won't have another chance to commit and I'll win in three turns. The only way a deck has a chance once the combo hits is to pretty much instantly take out the combo (this deck will gain incredible card advantage in a very small amount of time, and can generally assemble everything needed to prevent its own destruction in one turn or less). That's why I recommended in the above post saving up your support destruction and always having a Flooded Vault in play, so you can respond instantly to combo-breaking shenanigans.

Please do playtest it! This deck uses so many essential cards that I can really only good decks I can playtest it against are rush decks. It takes a few plays through to really get used to, though.

I think this deck could benefit from the new Day card silver twilight gets that stops responses and forced responses when it enters play. It can stop grasping cthonian from blowing up your ritual among other things...

Haha well this deck just got a lot weaker, to the point where it might not work anymore

Yup. Very disappointing. I'm not sure if it's salvagable.

I think you'd have to do some sort of Cthulhu/ST thing where the combo is just a fun thing you can do rather than the be-all end-all of the deck. You could still include the Shub Necronomicon though, which would be nice