Riddle: I heard of an invading, vanquishing army sweeping across the land, liquid-quick; conquering everything, quelling resistance. With it came darkness, dimming the light. Humans hid in their houses, while outside spears pierced, shattering stone walls. Uncountable soldiers smashed into the ground, but each elicited life as he died; when the army had vanished, advancing northward, the land was green and growing, refreshed. What is it?