Night's Whisper and/or Sign in Blood are almost always instant inclusions when I'm making an EDH deck with black. They're not really on theme, but don't underestimate how good turning 1 card into 2 is for just 2 mana. If you have good mana fixing you can also go for Insatiable Avarice which gives 3 cards for 3 and can tutor for extra mana. Cut a Deal and Secret Rendezvous are also quick card advantage but will even out with an opponent, though I think they work out in the user's favor. If you don't like that, Ancient Craving and Ambition's cost are only 1 more mana. Depending on how big your board gets, Minions' Murmurs can net you a lot of cards

Depending on how often you sacrifice creatures, Midnight Reaper or Morbid Opportunist might be worth it. I find Opportunist draws a lot more than you expect even with the once per turn limit since it triggers off your opponents as well. Welcoming Vampire and Enduring Innocence are also OPT but if you're playing/flickering small creatures a lot the cards will add up quickly.

People have already mentioned stuff like Deadly Dispute and Village Rites (both great) but since you're in Orzhov and seemingly fairly aggressive you have the chance to try Treacherous Greed, which I can't attest to personally but it seems like it could be worth testing