Sozin's Comet

Roku explains to Aang how 100 years ago, the Fire Nation used the incredible power of Sozin's Comet to begin the war with the other nations, and now, it'll return at the end of the summer, giving them the power to end the conflict once and for all. In other words, if Aang is to save the world, he must master all of the elements and defeat the Fire Lord within ten months.

Example of:
Race Against the Clock