Doomsday is a mirror-ish race where both decks are trying to combo off, with them holding up Force of Will and Daze; Thoughtseize lets us proactively strip their Doomsday or a protective counterspell before they can assemble their own kill. Boseiju, Who Endures is a land that can later be channeled for {1}{G} by discarding it from hand to destroy one of their artifacts (Lotus Petal, Lion's Eye Diamond) or a nonbasic land such as Cavern of Souls. It costs us the card and mana, but as an ability it can't be hit by Force of Will or Daze. We cut two lands and Song of Creation since we want to prioritize disruption and speed over value engines in this must-go-first race.