Cavern-Hoard Dragon's first ability counts only the artifacts controlled by the opponent who controls the greatest number of artifacts among your opponents. If you have an opponent who controls two artifacts and another opponent who controls three artifacts, Cavern-Hoard Dragon's mana cost would be reduced by {3}.
If the greatest number of artifacts an opponent controls is seven or more, Cavern-Hoard Dragon costs {R}{R} to cast.
Once a player has announced that they are casting Cavern-Hoard Dragon, no player may take actions to try and change the greatest number of artifacts an opponent controls before Cavern-Hoard Dragon's cost is locked in.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Antonio José Manzanedo
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Cavern-Hoard Dragon's last ability cares about the number of artifacts that player controls as the ability resolves.
