The Capitoline Triad

If a card in your graveyard has {X} in its mana cost, X is 0 for the purpose of determining its mana value.

Effects that modify a creature’s power and/or toughness without setting it will apply to creatures you control no matter when they started to take effect. The same is true for counters that change a creature’s power and/or toughness.

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 cards from your graveyard.

The Capitoline Triad’s mana value doesn’t change no matter how many historic cards are in your graveyard.

The ability of the emblem created by The Capitoline Triad overwrites all previous effects that set the power and toughness of creatures you control to specific values. Other effects that set these characteristics to specific values that start to apply after the emblem is created will overwrite this effect.

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.

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.

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.

The first step of casting a spell is to move it to the stack. If this causes the number of historic cards in your graveyard to change (probably because you are casting The Capitoline Triad from your graveyard), that new number will be used to determine the cost reduction.

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.

Once you determine the cost to cast The Capitoline Triad, you may activate mana abilities to pay that cost. If the number of historic cards in your graveyard changes while activating mana abilities, the cost to cast The Capitoline Triad remains what you previously determined.