The Plague: In the ancient pre-Imperial past on Eliéra, the Gray Contagion, a wasting disease with the same sort of morbidity and mortality as the Black Death.  It was responsible for one of the two great prehistoric population diebacks there (the one not caused by the astrobleme/ensuing Winter of Nightmares), and is thought to have originated as a leftover Precursor bioweapon.