Forgotten Title System story where a man with a door in the back of his head that pours out light when open

From what I can remember of the story a guy dies somehow and finds himself in a golden place. Everywhere is light, the light around him is knowledge. He reincarnates into an underground cave, I think he was naked too. Sometime after he arrived monsters came and so he killed them. As he killed one he felt a door in the back of his mind open, and light poured into his mind. He received a notice that he had leveled up. Gaining a level in pugilist I think. He levels several times over the course of the first chapter. I think it was the first chapter. Its bolded dialogue for the system notices, no blue boxes.