Orzhov Midrange leans on efficient removal (Fatal Push, Vanishing Verse, Get Lost) and disruption (Thoughtseize), so Dark Betrayal is a clean answer to their black threats like Sanctum Lurker or the 6/6 Demon token from Ritual Chamber, though it can't touch their white creatures such as The Queen of Dale or Skyclave Apparition. Fatal Push cleanly answers The Queen of Dale or an animated Mutavault, and with revolt on (a sacrificed Lander, Treasure, or Cauldron Familiar will do) it also reaches Skyclave Apparition and Sanctum Lurker, both mana value 3, before they generate value. The second set of Thoughtseize helps strip their removal or The Wandering Emperor before it can ambush your sacrifice fodder. Trim the thinnest value creatures (Greedy Freebooter, Umbral Collar Zealot, and a couple Shambling Ghast) since this matchup is more about trading removal and discard than racing with aggressive sacrifice synergies. Keep the core Cauldron Familiar/Witch's Oven/Mayhem Devil engine intact, as it still generates incremental damage and card advantage that outgrinds their midrange shell.