The Crematorians also known as the Levellers of Cities, the Wrath of Nagash Incarnate and the Ygnimortoi are a legion of the Ossiarch Bonereapers alight with the dark fires of the Shyish Nadir. They exist only to raze the lands of those that have earned the ire of Nagash. The sheer heat emanated by each of them can ignite hair and crackle skin and in their presence even stone and metal blackens and flakes away to ash. The fire within them burns so bright, that if viewed from afar, they resemble a sea of torches. As such, when struck down in close combat, each Crematorians can immolate itself in a detonation of arcane energy that takes it’s foe down with it - and if not, scars them for life. 
However, the wrathful magic within them is a double-edged sword. It consumes them from the inside out, meaning that no Ygnimortoi often exists beyond a week before the fire within it burns out, leaving their empty husk which is in turn reconstituted into a new Bonereaper. The exception to this is the leaders of the Legion, for they are built with enough care to last centuries without burning out - and even then, many amongst them have made pacts to tend to one another’s soul so that they may continue to serve. 
Their disposable nature is a source of great anxiety and dismay amongst these leaders. In order to find a solution to their condition, their field commander Ygnopatris Xaranos often forbids the burning of enemy libraries and storehouses of magic items until he has scoured each and every one for knowledge that might help preserve his legion from the balefire that burns within them.
The Ygnimortoi are made to kill and not to persist, hence they attack as a numberless horde of burning, skeletal killers that shrug off damage dealt to them. Once the Ygnimortoi have stormed the walls of the enemy city, annihilated its defenders and consumed it in the inferno that follows the ransacking, the next wave will sift through the ash and cinders that are left. With nerveless fingers they will pluck out the red-hot phylactery gems and blackened splinters of bone, bearing the remains back to their Mortisans so another horde of Crematorians can be fashioned.