I Am Really Not the Demon Lord!

"Awaken from the killing and become the strongest demon king!" In order to survive, young Fang Cheng was forced to sign a contract with a Slayer Demon. For each demon he hunts down, he can unlock a new ability. Facing this dark age of demonic invasion, Fang Cheng vows to fight back and become the strongest! (Source. Tencent, translated)