User blog comment:B214/Volume 21 info & spoilers/@comment-99.118.13.217-20160613135349/@comment-1359322-20160613142128

Permanently seal Trihexa. There's no such thing as permanent in DxD, even Gods grow old and die. So there's no permanent way to seal 666. Just like how Rizevim unsealed him, someone else will eventually unseal Trihexa again.