Here's an excerpt talking about the Demon King and immortality:
http://www.mangafox.com/manga/code_geass_nightmare_of_nunnally/v05/c026/10.html
http://www.mangafox.com/manga/code_geass_nightmare_of_nunnally/v05/c026/11.html
http://www.mangafox.com/manga/code_geass_nightmare_of_nunnally/v05/c026/14.html
http://www.mangafox.com/manga/code_geass_nightmare_of_nunnally/v05/c026/23.html
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You beat yourself up with your past. Don't blame yourself, blame the world. Blame God. Blame me.