User blog comment:B214/Volume 21 info & spoilers/@comment-71.15.106.32-20160504072650/@comment-99.20.60.169-20160504133828

The seal is temporay if you read between the lines in (high school dxd ex) they do get free pluse some time in the next,

30 years issei gains god class powers no BG needed. Now if you facter in god class power, pluse the BG Diablos Dragon

along with Great Red. There runs the posability that issei can kill Trihexa. On top of that open the way to meet new gods and godesses.

Heres a link so you can read for yourself (highschooldxd.wikki.com/wikki/user_blog:b214/high_school_dxd_ex)