I think that you went too far all-in on your combo with
Sanguine Bond, and that caused you to include some questionable cards. At present, I'd expect your deck to win out of nowhere occasionally, but to struggle otherwise.
Congregate, for instance, is pretty awful in the absence of
Sanguine Bond or a guildmage activation. It's decent with them, but still might not do much depending on the board state.
Champion of Stray Souls has little synergy with the rest of the deck and is expensive both to play and to operate. Something cheaper to either enable your combo or keep you
alive until it goes off would be better. Perhaps
Tithe Drinker (extort goes nicely with your theme) or
Blind Obedience?
I also think that
Hopeful Eidolon is questionable for you. It doesn't help your black devotion, it doesn't help you get your combo online to any appreciable degree, and any
lifelink creatures you make are unlikely to be very large to combo with
Sanguine Bond unless you start stacking them with Bestow in conjunction with
Herald of Torment.
I do like that you included some
Corrupt. You aren't mono-black, but the card is still powerful and swingy as a top-end spell. Consider
Liliana of the Dark Realms to help power out your swamps.
You might want to consider running a copy of
Diabolic Tutor to find either your
Sanguine Bond or (if you already have one) one of your big lifegain cards to close a game out.
Underworld Connections is excellent with Grey Merchant of Asphodel.
Orzhov Guildgates (or more expensive dual lands, but that can be burdensome) would be a good addition for you. I wouldn't want to reduce your
swamp count, but replacing a few
plains with the duals could help smooth things over for you.
Get a sideboard plan against aggro or look for ways to improve your maindeck against it. I think you'd have a lot of
trouble against that archetype.