During the height of the Dhakaani empire, interaction with exiles from the Aereni Line of Vol led to the duur'kala known as Iraala Dhakaan becoming the first dar vampire, a khaar'gaa. In the proceeding centuries, Iraala and her lover, Muurat, became the spymasters of the Dhakaani empire, serving as both internal security and also training a highly specialised commando unit who specialised in guerilla and psychological warfare. It was Iraala and the Kech Nasaar that first identified the threat posed by the Kapaa'vola, and it was Iraala's guidance that led the Kech Dhakaan to isolate themselves underground.
Lhesh's Iraala and Muurat remain the leaders of the Kech Nasaar, thousands of years later. They are likely the only individuals left who actually remember the empire before its fall. An undead creature cannot wear the imperial crown, the undead are severed from the Uul Dhakaan and are denied the wisdom of the past, but even so Iraala and Muurat were there, they remember the empire in a way that nobody else alive can. It remains to be seen what the years of true isolation have done to the Kech Nasaar, and if they still have the best interests of the empire at heart.
The Kech Nasaar were not idle for the last several thousand years. Their number has always been split between the living and the dead, and Nasaar Kol contains all the facilities required to sustain its mortal population. More importantly, every member of the clan that has died since their isolation has been preserved in a vast necropolis, their souls and organs stored in canopic jars, allowing them to remain dormant (and not draining the manifest zone) until they are needed. As such, the Kech Nasaar have a stockpile of thousands of undead soldiers that could be deployed with a few days notice.
The Kech Nasaar's use of sentient undead is a macabre mirror of the Aereni, and they do not allow their best and brightest to fade in Dolurrh. Some of the oldest undead in the clan were alive during the Age of Dhakaan, and have been filling their roles within the clan for thousands of years. Iraala and Muurat have always led the Kech, but there are other ancient khaar'gaa amongst the clan, and their study of necromancy has produced other varieties of undead, such as the forgemaster Denaal and dirgewraiths such as Hezra.
One particularly disturbing innovation of the Nasaar is the Maurezhi, a special form of ghoul that can assume the form (and memories) of any individual they have consumed. The disguise is imperfect, as it begins to rot away as soon as it is donned, but nevertheless Maurezhi have been seeded amongst the nations of Khorvaire.