: Shuffle your library, then exile the top card. Until end of turn, you may play that card without paying its mana cost.
in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.Urza, Lord High Artificer turns a collection of artifacts into both mana and a substantial creature. When he enters, he creates a Construct that grows with the number of artifacts you control. His mana ability then lets you tap an untapped artifact you control to add blue mana, giving even small utility artifacts and tokens a productive role beyond their printed abilities.
Sai, Master Thopterist is a natural partner because the Thopters it creates increase the Construct's size and can be tapped through Urza for mana. Cheap artifacts help establish the engine quickly, while Unwinding Clock can ready your artifacts during opponents' untap steps. The ability belongs to Urza and does not use a tap symbol on the artifact, allowing newly entered artifact creatures to supply mana this way.
His five-mana activation shuffles your library, exiles the top card, and lets you play that card without paying its mana cost until end of turn. This provides a use for excess mana, but normal timing restrictions and land-play limits still apply. It is a resource outlet rather than permission to cast every revealed card at any moment.
Urza works best when the artifact count is high enough that his entry immediately changes your available mana. Preserve a mix of card draw, interaction, and useful artifacts so the deck can function when he is removed. The Construct provides a combat plan alongside the mana engine, giving the same artifact development more than one payoff.
