Tainted Pact lets a black deck dig through the library one card at a time, choosing a revealed opportunity at the cost of exiling the cards passed over. It stops when you take a card or when the process exiles two cards with the same name. That makes deck construction unusually important: repeated basic land names can interrupt the search before it reaches the card you want.
Singleton decks can minimize that risk by using a mana base with distinct names, including appropriate snow-covered basics when useful. Even then, the cards you decline are exiled, so finding an answer late in the sequence can remove other important pieces from the deck. Evaluate the current need against what you are willing to lose before resolving a long search.
Thassa's Oracle provides the best-known combo partner in a deck with blue. With Oracle's entry trigger on the stack, a suitably constructed deck can use Pact to exile the remaining library by declining every card, allowing Oracle's trigger to win if its condition is met. Duplicate names, interaction, and changes to the relevant board state can disrupt the line, so the setup needs deliberate preparation.
Pact also functions as an instant-speed tutor without pursuing that combo. It can find interaction during an opponent's turn or a missing piece before yours begins. Its flexibility is strongest when the deck's naming choices support a deep search and you understand which exiled resources can safely be sacrificed to solve the immediate problem.
