MTL - Diary of a Sturdy Immortal - Chapter 1293 explain

A mysterious natural phenomenon caused the Earth to be plunged into darkness. The world turned into an endless bloody chaos and mankind is on the brink of extinction…

“Now that I’m back, I won’t allow my loved ones to die again!”

The Shadow Labyrinth – the most catastrophic existence humanity has faced in history.

Desir Arman is one of the six remaining survivors of mankind.

The six attempted to clear the final level of the labyrinth but ultimately failed, and the world came to an end.

However – when Desir thought he would meet his demise, what is revealed before him is the world… thirteen years ago?!

Desir is returned to the past, back to the time when he enrolled at the nation’s finest magic academy – Havrion. He is reunited with his precious friends, and is prepared to change the past to save the world and his loved ones…!

Three years remaining before the emergence of the Shadow World!

Change the past and gather powerful comrades to save mankind!

“I want to be a true demon.”

A boy not older than 20 years wished as he looked at the sky.

After suffering from the trauma for many years, he finally gave up on life.

People called him ugly, demon, and hideous.

He bared with it, but today he finally broke.

With a piercing pain in his chest, he died.

When opened his eyes, Logan found himself in another world and inside the body of a demon.

His wish was granted, but at what cost?

He had no power and was the lowest among the demons.

No one to help, no one to be with him.

But it did not affect Logan. Because he had something else with him.

‘Sin and Virtue System.’

Guiding him towards his goal to become a true Demon.

Logan’s conquest has begun, so be a part of it.

Otherwise, you will miss him.

The true Demon!

However, is the MC the 'only' one to have a system?

Hm?

Read the fucking novel and find out!

I, Hyoudou Issei, am a 2nd Year High school student and my age is equal to the number of years I haven’t had a girlfriend. And, someone like me got a girlfriend! I’m sorry buddies, I will walk the path of becoming an adult before you guys! – That’s how it was supposed to be, but why did I get killed by my girlfriend!? I still haven’t done anything yet! Are there no Gods in this World!? And, the person who saved me is the most beautiful girl in my school, Rias Gremory-senpai. I learned the shocking truth from her who isn’t a God but a Devil. “You have reincarnated as a Devil! Work for me!” Lured by Senpai’s breasts and treats, my life as a reincarnated Devil began. So the “Academy×Love Comedy×Battle Fantasy” starts here with just aggressive and worldly desires!