He transmigrated to another world and became a hero. The price of transmigrating and being reborn was that he had to defeat the Demon King and save the world.
A familiar setup and a familiar story. Lu Beisheng got it: he was going to be an invincible protagonist, overcoming one challenge after another while winning over beauties from every race, then defeating the Demon King and reaching the peak of life.
Some of the details were a little hazy. If he remembered right, the god who handled his reincarnation had said this was an S-rank world. So what? It was only S-rank. He'd once solved an advanced math problem while furious. How could saving the world be harder than that?
Then the other world welcomed him with an assassination attempt by the demon clan.
P.S. The cover is based on the design of one of the heroines. Image found online, all rights belong to the original creator.