Box sets are sets released in fixed packages (often one or more preconstructed decks) instead of through booster packs
Theres a good chance thats not worth doing in Deaths Shadow specifically, because we also draw Deaths Shadow itself less often if we make the deck bigger, and Deaths Shadow is much better than the other spells wed be adding
For one blue, one green, and three colorless mana, you get a 6/6 reach, trample, and indestructible creature that can't attack or block if it doesn't have an even amount of counters
At the very least, at every opponent's draw step, they draw a card
Relic of Progenitus, Preordain, and Skullclamp are all notably valuable because this deck contains their only retro-frame printing