LitRPG: I Evolved Demonic Beasts At Doomsday

*Two hundred years ago, the spiritual energy had awakened like an explosion in the earth. 
Countless animals and plants had evolved into a different Demonic Beast form due to their innate advantage.
...
With their extremely strong physical body and the and its speed of reproduction was astonishing, In a short period of time, it had formed a monstrous beast tide. 
Not only did it not fear ordinary weapons, but it also attacked the human cities without fear of death.
...
After losing 80% of the population and land, humans discovered that many cultivation techniques that they could not cultivate in the past could now be cultivated.
Except for a few people with exceptional talent, most people's cultivation speed was too slow. 
There was no way to effectively resist the beast tide, so they could only build the Base City. It was used to protect ordinary humans.
...
As a poor boy three days ago, in order to protect his sister, he offended the young master of Loong family, one of the three big families in Songbei Base City.
He had been coerced and enticed by the Mission Department of the base. 
They had forcefully recruited him to investigate the situation of the Demonic Beast in Northern old city.
In this post-apocalyptic world, if he did not do any missions, he would not have food to eat. Therefore, he had to come.
...
On this mission, There were a dozen Immortals who were with him. Because they disliked him being an ordinary person, they were unwilling to bring him along.
He knew that he was a burden, so he only dared to follow them from a distance. 
However, when he encountered a small wave of beast tide, he personally witnessed all of them being eaten by the Demonic Beast.
And now he knew he wouldn't have been able to escape death.
...
"Ding... Super Demonic Beast Synthetic System binding."
"10%... 50%... 100%. Super Demonic Beast Synthetic System Binding Successful. Received Newcomer Gift Bag. Do you wish to receive it?"
"Congratulations to host for obtaining the cultivation technique, Demon Subduing Curse. As long as host uses the Demon Subduing Curse on a Demonic Beast without any resistance, you can keep the Demonic Beast as your own Demonic Beast."
Now in this Cruel world with such power could him protect his sister and became the immortal?