Terrageddon builds card advantage with Sylvan Library before Armageddon-ing the board and grinding you out with Terravore and Treetop Village, so Xantid Swarm both pressures them early and shuts off their ability to respond on your attack step. Tormod's Crypt shrinks Terravore by exiling a graveyard, since its power and toughness equal the number of land cards across all graveyards, directly weakening their best late-game threat. Tranquil Domain cleanly answers a resolved Sylvan Library since you run no enchantments yourself to lose to the sweep. Crystal Vein and Tsabo's Web come out because you want cheap, reliable mana for a post-Armageddon recovery rather than lands you sacrifice or narrow utility-land hate.