Ub Reanimator wants to cheat Griselbrand or Atraxa into play via Animate Dead, Reanimate, or Shallow Grave, so we need fast pressure and ways to exile their graveyard before they can reanimate from it. Dauthi Voidwalker exiles any creature cards that would go to their graveyard, directly cutting off their reanimation targets, and is a quick evasive clock. A second Fatal Push helps answer their reanimated threat if it somehow has mana value 2 or less after -1/-0 from Animate Dead, or more realistically answers their early Dark Ritual-enabled plays. Stitcher's Supplier actually helps fuel a reanimation target if they ever interact with our graveyard, and Wail of the Forgotten is too slow, so both come out.