To determine the total cost of a spell, start with the mana cost or alternative cost you're paying (such as a spectacle cost), add any cost increases, then apply any cost reductions. The mana value of the spell remains unchanged, no matter what the total cost to cast it was.
A card's spectacle cost is the same no matter how much life your opponents lost or how many opponents lost life.
Damage dealt to a player causes that player to lose that much life.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Slawomir Maniak
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Spectacle cares only that an opponent lost life during the turn, not that the opponent's life total is currently lower than it was. For example, if an opponent loses 1 life and then gains 2 life in the same turn, you can cast a spell for its spectacle cost that turn.
In a multiplayer game, if an opponent loses life and later that turn leaves the game, you can cast a spell for its spectacle cost. (If a player leaves the game during their turn, that turn continues without an active player.)
Spectacle doesn't change when you can cast the spell. For example, you can't cast a sorcery with spectacle during an opponent's turn unless another effect allows you to do so, even if that player has lost life this turn.
