we've only seen one demon kj and arch.
but we've seen at least one person who had become a demon in the main universe alive and well in the alternate, othaar. there's also a soul echo(to be fair, we have no idea what these even are) of othaar on mac'aree. othaar, by the time we reach him, hadn't become a demon, but had pledged himself and taken in fel. he transforms right in front of us.
socrethar was killed in netherstorm in the MU. he could have been permanently killed, but we can't be sure, so seeing othaar transform may not be entirely indicative of the process, but it's the only observation of two instances of a demon's existence.