Dark Star Emperor

Dark Star Emperor manga summary:

When Su Xiaobai is dying from an illness, he accidentally comes to another world with the remaining soul of Long Ming, Star King, the strongest human in this world, in his body. He gains the strongest ability called “Infinity of Darkness” at the price of feeling terrible pain whenever he gets an erection. Only when he finds out the person who plotted Long Ming’s death and succeeds to the throne can the pain disappear…