A guy from earth died, reborn as a son of the ruler of the demon realm. I believe he has 1 older brother a sister and another who is sealed in the realm. He leaves the demon realm to a neutral human territory town, created by dwarves and humans on top of a ruin. I remember him using a god(not sure if it really was) power to create the codes for the internet system, he created hearthstone, and a few other things as well. I had read it on uukanshu -Edit- Nevermind I have found it, for those interested its below. Cultural Invasion in a Different World https://www.uukanshu.com/b/59450/