the expansion of the burned lands, caused by malefor's resrection of the distroyer, is the breaking point for terrador. he knows that the distroyer is coming, and if it is able to cross the belt of fire, as the valley of avon is now called, it will be too late to save the world. cynder's the one to ask how deep the canyon is before silver notices her plan- flood the valley and slow the distroyer and give the elders time to distroy it. when you do topple the avon dam, the distroyer is stuck in the flood waters long enough for silver and cynder to attack the distroyer and distroy it's dark crystals. then it breaks free and dies as it reaches the other side of the valley.