At some point in the Nur'xal's history, they were visited by the 'Scaleless Ones' (Pretty much some of the races from the prior cycle... expect Krogon) who enlightened several Nur'xal tribes about the 'Void Gods' (Reapers) and the threat they posed. They didn't have one of Liara's Black Boxes but they had put together something that gave a fairly decent idea of what the Reapers did (Using the Mass Effect to trick races into follow a path that they can easily reap, the corrupting nature of Reapers and their technology, Husks, ect.) and the fact that they were waiting somewhere in the blackness of the void, prepared to wait for the chance to strike. 

While the original tribes that learned this would be wiped out during the 200 year long First Crusade, they left enough behind for the victors to acknowledge the threat and figure out a way to prepare for it. The arrival of the Void Gods is considered to be Nur'xal version of Ragnarok; The battle to end all battles that will either reshape their way of life or end it outright if they are too weak to win.