Behemoth of Vault 0

Some triggered abilities that state that you “may pay” a certain amount of energy describe an effect that happens “If you do.” In that case, no player may take actions to try to stop the ability’s effect after you make your choice. If the payment is followed by the phrase “When you do,” then you’ll choose any targets for that reflexive triggered ability and put it on the stack before players can take actions.

Some triggered abilities state that you “may pay” a certain amount of energy . You can’t pay that amount multiple times to multiply the effect. You simply choose whether or not to pay that amount of energy as the ability resolves.

When you pay energy equal to the target nonland permanent’s mana value during the resolution of Behemoth of Vault 0’s last ability, a second “reflexive” ability triggers and is put onto the stack. This ability does not have a target. Each player may respond to this triggered ability as normal.