The Ghouls' Conquest (of the Mirror Universe) is the tragic event that took place in the week of unconsciousness for the lost heroes after the Neo Ghouls Arc. It is unspoken and can only be read here.
After Cisaderes blew up the heroes, most of them being captured later or blown off to Earth, he set out to conquer the the rest of the planet. His first stop was the Hollows' base in that universe; TKHM, however, had made everyone retreat to Earth beforehand, so it was futile. Cisaderes then invaded the prison and took everyone there prisoner. He then sent out grunts to take out the other areas of the planet while he goes onto other planets in the galaxy.