UBG Reanimator wins by discarding Griselbrand, Atraxa, Grand Unifier, or Archon of Cruelty and bringing them back with Reanimate, Animate Dead, or Shallow Grave, so Leyline of the Void is our best answer — free from an opening hand, it exiles their discarded or milled threats before those spells have a target, and it only affects cards headed to an opponent's graveyard so it never disrupts our own dredge or Bridge from Below plan. The extra Unmask lets us proactively strip a reanimation spell or their own Unmask before they can set up. Bridge from Below is unreliable here since we're racing a deck with a much bigger top end rather than trading in combat, so we trim it for the graveyard hate package. This matchup is close to a race, so prioritize denying their reanimation targets over grinding.