Fusion Overlord at the beginning, kill the enemy and become stronger

Xu Qianqiu travels to a world where people and demons coexist.
At the beginning, he became the eldest prince of a kingdom. Who would have thought that the kingdom was facing the invasion of demons, and more than [-] cities were destroyed in a row. The king actually asked him to lead troops to attack the demons.
Fortunately, the strongest enemy killing system is activated, integrating the power of the overlord.
From then on, there is one more crazy killing god in this world!
"Congratulations to the host for slaying the demon and obtaining the Eighteen Riders of Yanyun."
"Congratulations to the host for slaying the demon and obtaining the Tiger Roaring Dragon Roaring Iron Shirt."
"Congratulations to the host for killing the demon and obtaining the character card Li Cunxiao."
…………
Xu Qianqiu led the army to destroy the demons and regain lost ground, but just when they were about to hit the demons' lair, the imperial court signed a peace agreement with the demons and ordered him to call gold to withdraw his troops.
Xu Qianqiu looked at the direction of Wangdu with a faint gaze. You must know that he can not only kill demons and get system rewards... He can also kill people! !

Key words of the novel: Fusion Overlord at the beginning, you will become stronger when you kill the enemy