UBG Reanimator wants to Dark Ritual or Shallow Grave/Animate Dead a huge threat like Atraxa, Grand Unifier or Griselbrand into play as early as possible, so Disruptor Flute naming Animate Dead, Reanimate, or Shallow Grave taxes their key reanimation spells by three mana, which can push them out of range on an early turn. Harbinger of the Seas provides an early blocking body and pressure while we race or disrupt; our maindeck Tormod's Crypt already attacks their graveyard plan, so we lean into additional disruption rather than doubling up on grave hate. Sink into Stupor is weak against a deck that just wants to resolve a reanimation spell on a creature already in the graveyard, since bouncing the reanimated threat doesn't exile it and they can simply recast their cheap enabler. Pithing Needle has few good targets here since their key spells aren't activated abilities.