The Immortui is a complex ritual of Black Occultism, requiring several dark occultists to gather at a crossroads and ritually sacrifice a human child. The fragmented soul of the child will then travel to the closest thirteen corpses, returning them to a semblance of life with which they can do nothing but obey their creators.
The ritual can be repeated more than once, and each sacrifice will be sufficient to invoke thirteen undead corpses, known colloquially as zombies.
It should be noted however that every living human bitten by a corpse will rot slowly and lose their own willpower, transitioning into an undead ghoul themselves. Freshly turned zombies are incapable of transmitting the black spell with a bite, however, and they are differentiated from their kind by the color of their eyes; the original thirteen zombies have pure white and glowing irises, where as the second wave are blinded and weep blood. Bitten sorcerers lose all magical ability, and elementals rot in their true form and lose their connections to the elements.
Extremely hard to kill, the cadavers can however be halted by salt, a large quantity of which will eat through the spells that maintain them and return them to lifeless corpses. Applying a sufficient amount of the substance without being bitten is no mean feat, however...