Pact of Negation counters a spell without requiring mana at the moment you cast it. That makes it useful for protecting a decisive play when your available mana has already gone into a threat or combo. The delayed cost is significant: at the beginning of your next upkeep, you must pay three generic and two blue mana or lose the game.
A combo deck can use Pact to answer the counterspell or removal spell that would stop its finishing sequence. A control deck can also hold it as emergency interaction when tapped out, provided it has a realistic plan for the next upkeep. Its target can be a creature or noncreature spell, giving it broader coverage than counters restricted to one card type.
Unlike Force of Negation’s alternate payment, Pact can be cast for zero mana during your own turn. That difference matters when you are trying to resolve a sorcery-speed engine or protect a creature entering as part of a combo. The tradeoff is moving a large mana commitment into the next turn rather than paying with another card immediately.
If Pact resolves, the upkeep obligation applies even when the targeted spell could not be countered. Being unable to produce the required blue mana can be fatal despite having enough total mana. Opponents can also respond to Pact with further interaction, so it is not a guarantee that the finishing attempt will succeed. Plan both outcomes before casting it: the turn where your sequence works and the next upkeep if the game continues.
