UB Reanimator's entire plan hinges on getting Griselbrand, Archon of Cruelty, or Atraxa into the graveyard via Faithless Looting or discard and then reanimating with Reanimate, Animate Dead, or Shallow Grave, so Leyline of the Void, which can come down for free in the opening hand, completely shuts off their reanimation targets by exiling cards before they ever hit the graveyard. This is one of the single best sideboard cards against this archetype and should come in as close to a full playset as possible. We trim the enchantment/artifact-removal creature that has no relevant targets in this matchup. Keep Thoughtseize to fight their own discard and disruption, and hold priority with instant-speed Solitude/Swords to answer anything that slips through.