The Punishing

It is the year 2481, and Satan has conquered Earth. After rejecting their Messiah and straying from the word of God, humanity falls under complete control of Satan and his minions - a race of artificially modified humans, who no longer call themselves Homo Sapiens, but rather, Homo Demonicus. Satan keeps the entire human race in captivity as his slaves, but the slaves don’t know they are slaves until they graduate from high school. Upon graduation, humans are put through The Punishing where they are accused of all their sins from birth until graduation, and punished most torturously. The humans who survive The Punishing then become slaves to the Homo Demonicus for the rest of their lives.
This illusion that humanity is sovereign also allows Satan to observe each human from birth until high school graduation to separate the good humans from the evil humans - the evil humans become Homo Demonicus, while the good humans go through The Punishing and become slaves. God allows The Punishing on humanity because humanity fell so far from morality in the 21st and 22nd centuries that He became ashamed of His creation. As Satan starts breaking his own laws and his kingdom begins breaking down, it starts to look as though humanity might soon become extinct. This causes God to pity humanity, so He sends a being from the most advanced species in the universe to Earth to help a human slave named Rex give humanity a chance to survive.
This novella by author Justin Mohn is just over 100 pages and is intended for mature readers only due to content containing violence, gore, sex, and profanity.