I Became the Demon Lord and my Territory is an Uninhabited Island

I Became the Demon Lord and my Territory is an Uninhabited Island, I Become the King of Darkness and My Territory is an Uninhabitated Island Maou ni nattara Ryouchi ga Mujintou datta 魔王になったら領地が無人島だった novel:

Our protagonist Caam(?) is a man from Earth that was reincarnated into this world as a demon. In order to survive, he spared no effort in avoiding the sparks of battle and without noticing it, he became strong himself.
Then, one day a demon that is stronger than himself suddenly appeared in front of his eyes. The demon said「You seem quite strong, aren’t you? Don’t you think someone that strong should become a Demon Lord?」. He tried to negotiate but, there was no right to refuse.