Demon Dragon system DDS

(I will be using this synopsis for the mean time, till I think of something more captivating, but I assure that this is a promising novel so give it a try).
***
"Hunt or be hunted," is the only way for humans to survive in this world.
Monsters are evading the city, the world has merged, and portals sprout everywhere.
To prevent the invasion of monsters, humanity must clear these portals on a regular basis.
If humanity is successful in killing a creature in the portals, they will be granted special abilities and can use their core to upgrade their levels.

Christopher's first portal adventure turned into a nightmare as he was tortured by monsters.
He miraculously wound up in a hospital.
[Ding...You have gained a system.]
Christopher used the system to strengthen himself, only to discover that the world is gearing up for something far more massive.
Will humanity perish if Christopher fails to defeat this unknown threat?