They originated from one of the Milky Way's satellite galaxies. Specifically the Large Magellanic Cloud. They initially started off as Precursors who rendered themselves into a dust that could regenerate their past forms through unknown means. Ten million years passed, and time rendered the dust defective. When treated on living creatures, the dust caused abnormalities and abortions. Eventually it would compel the affected to consume their own. Abnormalities and mutations increasingly became worse and worse until the Flood as we know it was born into existence.