User blog comment:B214/Question & Answers 5/@comment-178.128.187.173-20150905210811/@comment-3290441-20150927230955

The curse was the constant buildup of hatred generated from the numerous Boosted Gear wielders and probably started by Ddraig himself. If you're referring to the anime then it was curse placed on Issei by Loki.

In both cases they've been resolved already, in the first using the Divine Dividing fragment Issei stole and the other it was removed by Ophis.