Beseech the Mirror
Black tutors have almost always ended one of two ways: the card goes to your hand and you spend a turn casting it, or you pay a steep life cost to skip a step. Vampiric Tutor and Demonic Tutor set the poles, and everything since has negotiated the tax between them. What makes this one different is that Bargain folds the tutor and the payoff into a single act. Sacrifice a token, a Treasure, a spent enchantment (something you were done with anyway), and the card you fetch does not go to your hand: it goes onto the stack, cast for free, provided its mana value is four or less. That last clause is the whole balancing act. A tutor that also casts what it finds would be a two-card combo compressed into one, so the design walls it off at four, which is high enough to reach a devastating body or a game-ending enchantment but low enough to keep the most abusive endgames a mana-value or two out of range. The Bargain cost is what pays for the compression: you are not just spending four mana and a card, you are spending a permanent you already control to skip the tutor's usual tempo penalty entirely. Without the sacrifice it is still a fine tutor that hands you the card, which keeps the spell honest when you have nothing to feed it.
