Undead

Though they exist in undeath they still are not truly eternal, while their flayed souls allow them to be forced into vessel after vessel, as dry empty husks, it takes a great deal of necromatic energy to maintain ones will and ego, Once an ego has truly faded away it can never be recovered. Fell rituals and dark arcane tools are used to harvest these souls, which are then consumed by an undead. Rituals and tools which lay buried beneath the sands but may yet be learned an unearthed Should the soul of a creature be consumed in this way by an Undead they would effectively extend the length of their "life" and allow them to hold onto their minds and personalities. This would be reflected by a number of years equal to the hit dice of the creature(s) used during the ritual, rolled, totaled, and subtracted from your age, (the average of all the dice can also be used if rolling to many dice is an issue). However only humanoids bear the souls compatible for siphoning.
Undead also carried with them smaller tools used to extract souls but on a much smaller, unrefined scale, although rare these allowed an Undead to siphon a soul directly from creature within 30ft of them upon their death, assuming that they were the ones who caused that death. These tools varied in design fron haunting lanterns, to cages, even lamps. A souls siphoned in this way only subtract a total equal to one of its own hit dice. It has been noted that those with more magical potential and ability gain less from such a process, perhaps the arcane art strains their lifeforce, yet this is still a matter of debate.
An undead can also enter a special kind of long term dormancy, having to be entombed with proper ritual and procedure, effectually causing them to age at a slower rate. For every 10 years that pass in this state your mind only decays as if 1 year has passed.