The Imiriaran Cycle
The word “firstborn” has two important meanings in the Imiriaran Cycle.
Firstborn (Proper Noun)
The Firstborn was the first child of God. He created the plants and all the living things in the world. After God denounced him for his Pride, he rebelled against God and fell from Grace. This is the Original Sin, and it taints all firstborn sons. The Firstborn invented evil and is the source of all that is awful, despicable, and corrupt in creation.
Especially humans, whom he created.
Firstborn (Common Noun)
The firstborn is the word that collectively encompasses the firstborn male children of the world. Due to the Original Sin of the Firstborn, all firstborn are corrupted from birth, bred and born to evil.
Because of this, all firstborn sons (and many firstborn daughters, especially in poor or fundamentalist households) are given to the Church at the age of ten. There they are trained to be clergy, with the goal of purifying them of sin and restoring to them their free will. Because of this, the Church is made up almost exclusively of firstborn “sinners.”
There is still debate among scholars over whether the taint of Original Sin is really as terrible as doctrine says. Still, many of the most horrific human beings in history were known to be firstborn (at least anecdotally), and so few dare buck the system with their child.
Because of the taint of original sin, the youngest son inherits in Imiriaran society, as he is the farthest removed from the taint. This makes fratricide a rampant issue, as killing a younger child is typically far easier than an older one. In fact the Valerian heir apparent is raised with round the clock bodyguards to protect him from his older brothers. These men typically become the core of his royal guard when he takes the throne, and this custom is practiced in many other countries as well.