Longtusk Cub

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.

If a spell or ability with one or more targets states that you "may pay" some amount of energy , and each permanent that it targets has become an illegal target, the spell or ability won't resolve. You can't pay any energy even if you want to.

Some spells and abilities that give you energy may require targets. If each target chosen is an illegal target as that spell or ability tries to resolve, it won't resolve. You won't get any energy .