Once you determine the cost to cast Excalibur, you may activate mana abilities to pay that cost. If the total mana value of historic permanents you control changes while activating mana abilities (probably because you sacrificed one or more historic permanents), the cost to cast Excalibur remains what you previously determined.
Lands are never cast, so abilities that trigger “whenever you cast a historic spell” won’t trigger if you play a legendary land. They also won’t trigger if a card on the battlefield transforms into a card with the legendary supertype, the artifact card type, or the Saga subtype.
Once you announce you’re casting a spell, no player may take actions until the spell has been paid for. Notably, opponents can’t try to remove historic permanents you control from the battlefield at that time.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Thanh Tuấn
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An ability that triggers “whenever you cast a historic spell” doesn’t trigger if a historic card is put onto the battlefield without being cast.
A card, spell, or permanent is historic if it has the legendary supertype, the artifact card type, or the Saga subtype. Having two of those qualities doesn’t make an object more historic than another or provide an additional bonus—an object either is historic or it isn’t.
Excalibur’s mana value doesn’t change no matter what the total mana value of historic permanents you control is.
Some abilities trigger “whenever you cast a historic spell.” Such an ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.
