Moto Sekai Ichi'i no Sub-chara Ikusei Nikki - Chapter 30: Convenient Delusions

Hao Yan Che is an immortal and Yuan Gun Gun is a mortal.

A skilled but unfortunate modern-day assassin was killed in a freak accident. However, he soon found himself summoned to another world by an evil wizard, who had him possess the body of a boy as an experiment. After successfully planning the evil wizard’s downfall, he used the wizard’s body as a springboard to gain influence and power in his new world.

- Description from Novelupdates

When a traverser mistakenly thought that he had crossed the back of Britain in the 1980s, and when he was suffering from childhood, he would eventually sink into the darkness in repentance, and a strange and familiar admission notice would light up his life.

But when he came into contact with the deeper darkness, did he choose to stay away from the abyss or jump into it without hesitation?

”You wait, I will be back soon.”

On the fifth floor of the St. Mungo Hospital, he finally firmly held her pale hand.

This is a story of biting teeth, even a liar that you can lie, and struggles repeatedly in pain and tenderness.

- Description from Novelupdates

There are three realms in the multi verse; The Cosmic, the Mortal and the Nether realm.

Hal grew up in the mortal realm with no idea whatsoever of who his parents were.

His love for relics, adventure and the occasional danger would aid him in finding a genie's lamp.

Rubbing a genie's lamp should grant three wishes, but Hal is struck by lightening instead and eventually arrives in the Cosmic realm with a large book in his mind space titled; The Primordial Cosmic Grimiore, that apparently holds all knowledge for a perpetual reign.

And so Hal's journey begins in a little city called Salmon city, in the body of a 17 year old boy also named Hal but was sold into slavery by his own father.

Hal finds out who his mother (prior to reincarnation) whose bloodline now flows unmixed in his new body is....

Join Hal as he battles mortals, Dragons, phoenixes, Gods and many more together with his harem of Beauties for the complete subjugation of the three realms.