I forget the title, and even the characters' name. This manhwa started with a boy getting killed then resurrected by a princess of another world (demon, if I'm not mistaken). Then, she brought him to her world, but he had to leave his sick brother behind. Later, it was revealed that his brother got dragged to that world too, but he became possessed by something. This manhwa has a sequel and the boy became a ruthless king, but the main character seemed to be a white-haired boy (supposedly his son). He also had a queen whom he didn't love, royal son and daughter. The princess before was then imprisoned, but she was still loved dearly. The brother now has another powerful force to interfere with the king's objective.
The royal son was thought to be born from an affair, but the chapter I was reading revealed that he truly was the king's son.
If you need more information, I'll try to summon it from my memories. I just can't remember the title.

Been awhile since I've last read it, but could it be Immortal Regis?

Yes!! That's it! Thank you so much!