Undeath and the Undead

The greatest necromancers of the titanic age were Zerak and Sharn. Zerak of the Pale Bindings was the grandson of the City- Father of Thosis and a master embalmer in the Hall of Somnolence. Sharn Gravesinger was a chantress in the Grey sepulchre. Both were scholars in the Conservatory of Eternal Silence, where his work on the animating principles of the body paralleled hers on the nature of the soul. Their collaborations were the first work to prove the existence of necrosis and its similar­ity to the entropic and revenant energies of the Grim Plain. They are also believed to have created the first revivification spell.
As the Colossus War stretched out, Zerak and Sharn attempted to reanimate slain Titans to rejoin the fight, and were convicted of blasphemy and betrayal of the duty of the necropolis. Zerak was imprisoned, while Sharn, lacking Zerak's familial protections, was sentenced to death. Zerak lashed out in rage at this injustice. Wielding their forbidden magics, he forced his way to Shawn's cell, where together they unleashed the curse of undeath which binds the city to this day.
The curse unleashed by Zerak and Sharn destroyed the population of Thosis and transformed many of them into grotesque mockeries of life, the first undead. It also changed the very nature of the World, introducing undeath as a possibility. For a long time, the full implications of the curse were not realised, but then came the Cataclysm and the deaths of four of the seven Primordial Dragons, the oldest beings in Existence. One of the slain Primordials, Tropos, became undead, and more than that, a god of Undeath.
Undeath is an unnatural state caused by the animation of dead tissue by or spiritual manifestation through the medium of Necrosis, or necrotic energy. It occurs when some part of a dead creature becomes saturated with necrotic energies, causing that part to become animated by a parody of life. The existence of such things is an abomination, but not so ontologically harmful as the Outerfolk. Instead, the presence of the undead tends to weaken the boundary with the Grim Plain, resulting in chills, fog, and areas with a disquieting air as necrosis pools.
There are various types of undeath, differentiated by the degree of necrotic infection, and the manner of Binding Principle that controls them.
Simple undead are those created when a single part of a deceased being is infected by necrosis: the bones of animated skeletons; the flesh of zombies or ghouls; the spirit of the various forms of ethereal or spectral undead. Such creatures have little intelligence and operate on a very limited set of basic imperatives or lingering habits. This group also includes rarer and more outlandish undead whose blood or skin is necrotised.
Greater undead is the term given to those who have necrotic pollution in several parts of their self. More powerful corporeal undead might have necrosis in their flesh and bones, while an infection to spirit and blood creates undead with corporeal form and spiritual power, like the bodak and the dullahan.
A very small number of undead bear necrosis in every part. This usually only happens as a result of powerful and deliberate magics, such as those which permit a spellcaster to become a lich. Alas for those who seek such immortality, the flesh eventually decays, leaving only bone and spirit, too little to bind a complete personality.
Aiaos does not have vampires. Their form of blood infection has never reached this Existence.