the Rulers and Sovereigns are two factions created by a powerful godlike being to be in constant conflict against one another. Rulers were made to create and defend civilization while Sovereigns were created to raise armies and destroy the Rulers.
Sovereigns have chosen Earth as the next battlefield. So, Rulers are sending “monsters” to earth in an attempt to strengthen enough humans so they survive the upcoming war between the Rulers and Sovereigns.
I could go on but I just gave enough to get over the cliff rather than spoil the whole series.