Grixis Reanimator relies entirely on the graveyard: Unearth and Persist return creatures directly from it, and Abhorrent Oculus requires exiling six cards from the graveyard just to be cast. Rest in Peace shuts all of this off simultaneously, denying Unearth and Persist targets and starving Abhorrent Oculus of its additional cost. Consign to Memory picks off Psychic Frog or an Archon of Cruelty cast for its colorless-adjacent value triggers, while Mystical Dispute is cheap insurance against their blue Spell Pierce or Thought Scour. Land Rest in Peace as early as possible, since this matchup collapses for them without their graveyard plan.