Thread:Merkarion/@comment-36663391-20181020210134/@comment-37165251-20181020220118

In DxD Zero is said that Ajuuka is researchin Agreas (where the Crystals are which he used to create the Evil Pieces) shortly before the Civil war (he was 60 at that time).

So i would say shortly after ending the Civil War.

Rivezim was hidden all the Time because he had no interrest in Fighting. Only after, through Issey and that Oppai God^^, it was confirmed that there are Other Worlds he became active as he found a motivation.