Loki's curse

Kurenai spoke.

"……Yes. Having broken out of prison, Loki, in order to get revenge upon us who he saw as part of the sect of Rias kaa-san and the Vanishing Dragon Vali belonged to, plotted a retaliation. For that reason alone —— he placed a curse on Asia kaa-san."

Zen continued.

"……We were overcome with grief. That day, since there wasn't anyone in the vicinity of Asia kaa-san, she became the recipient of his revenge……and fell into a sleep. A deep sleep that she cannot wake up from. ……It seems to be a curse that repeatedly invokes a countless number of atrocious technical equations that Loki developed while in prison, and even with numerous magical experts, lead by Rossweisse kaa-san, they concluded that unless Loki dispels it himself, or else explains the technical equation, Asia kaa-san will not wake up."

Kurenai continued at this point.

"There was an analysis that maybe if an immense amount of time was put in to it, it could probably be reverted however she ...it's not that significant. Asia kaa-san is already starting to weaken because of the curse, so she won't last until the analysis is complete. If that's the case, considering the approach of defeating the 'UL' that traveled through time with Loki and securing Loki himself, a discussion of the higher-ups concluded that this was the most realistic approach for dispelling the curse, and thus we are now executing said tactic."

High School DxD EX, Chapter 2