User blog comment:B214/Volume 21 info & spoilers/@comment-99.118.13.217-20160610101418/@comment-99.118.13.217-20160612210637

I think the writer should of saved Trihexa as the final villain and have Issei & Vali kill it or have the final villain be from The Other World. "I know the writer said Highschool DxD series won't cover that" but I think it should be part of the final story arc.