User blog comment:DeathGodPrayer/Talk About What You Want!/@comment-213.106.131.123-20131031152845/@comment-213.106.131.123-20131101101138

That is the most plausible theory out there in my opinion. What ability would Issei get if he took in Tannin's power (the reason I used Tannin is because we know the most about his abilities)?