Who is the man shown in the last cutscene after beating Diablo?

I've just finished diablo 1 again, and something intrigued me.

After beating diablo, your hero remove the soulstone from his forehead (with a dagger), and after that he become human with a hole in the head.

Who is he ?

Image :

I have tried to find this on the internet but nothing...

If someone knows it would be cool :)

Link to the video