User blog comment:FrankieX98/The future members of the Issei's harem and peerage./@comment-37165251-20181209183528/@comment-37165251-20181209203354

@Grandpiece as far as it is translated it is said that Lord Nebiros is the only one of the six Lords (the other fice did die in the Civil War) that is knowing about the existance of the Malebranche. It seems only the four Original Maous and the six Lords of the Houses of Lucifer did know about them.

Maybe this will change when DxD Zero is complete translated, but this is the actual knowledge.