All alduin does is bring other dragons back to life. Smaug died beacuse he was standing still and the chances of you hitting smaug in the weak spot is 0.000000007
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Um smaug doesnt have a weakspot you just have to shoot him with a black arrow that spot was just a spot penetrated by another black arrow making it a better spot to attack
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So that was his only weak spot
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Its not his weak spot you can still penetrate other spots of his hide with black arrows but that spot was penetrated by a black arrow and woulfnt it be better to hit the spot already weakened than shoot 2 arrows in the same spot or riddle him with arrows plus alduin can only be killed by the dragonborn
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Well that was his weakest spot. And screw dragonborn if a human can kill alduin smaug can.