The Age of Sacrifice

The Age of Sacrifice was the era that Nihilo was born in.  It came before The Age of Rebellion.
This age features heavily in Nihilo's backstory.  During the age of sacrifice the Avatar struck powerful deals with mortals.  In this time the world had much less water; the world was flooded when a well-intentioned doctor struck a deal to bring rain to the world and make water freely accessible.  This was accomplished by opening a portal to the Elemental Plane of Water.  The age was eventually brought to an end by some band of heroes, but Nihilo was not aware of the details.