UBG Reanimator wants to discard or ritual into Atraxa, Griselbrand, or Archon of Cruelty and bring them back with Reanimate, Animate Dead, or Shallow Grave, so Leyline of the Void, ideally free from your opening hand, exiles every card that would hit their graveyard and shuts off their own graveyard as a reanimation target. That makes it close to a must-answer piece of hate. Be aware that Reanimate and Animate Dead can target any graveyard, so keep an eye on your own graveyard too. Massacre stays in the sideboard: their creatures are huge reanimated threats and 3/3 Koma's Coil tokens, and -2/-2 doesn't kill any of them. Hogaak gets cut because it is hard to cast without a board of creatures to convoke, and Grist is trimmed to make room for the full set of Leylines, since raw disruption matters more than value against a deck trying to cheat in a single giant threat. Witherbloom Command stays in because its mode that destroys a noncreature, nonland permanent with mana value 2 or less can hit Animate Dead. Keep Thoughtseize in as your best tool to strip their Reanimate or Animate Dead before they can use it.