Time is the enemy of all things mortal. There are creatures who dwell amongst us who deny their mortality, but they are as vulnerable as any. The blood craving creatures of the northlands can be defeated by sunlight and a restriction of their diet. The slumbering guardians of the desert can be reduced to smoldering ash. And, the wizards of Babylon risk destruction by time and decay.
There are artes in which a practitioner of magic may attempt to cheat his natural death by confining his soul in his own corpse by use of a magical box known as a phylactery. This device prevents the soul from continuing to its proper place, lengthening the amount of time available for study and learning. This device is essential to transform the living wizard into the abomination known as a lich.
But, it does not guarantee immortality. As the lich detaches itself from the needs and desires of humanity, it has a tendency to neglect the maintenance of its own body, working instead to greater efforts in research and study. As the lifeless creature learns more about the theoretical, the theoretical becomes more substantive while substance fades into the theoretical.
The lich, by nature, risks losing perspective of reality. As the lich begins to contemplate and meditate on the things it has learned, it finds little reason to consult even its extensive collection of tomes, relying entirely on the knowledge it has compiled and deduced. Eventually it loses interest in mundane material matters and does nothing but contemplate the things that are beyond the matters that are inconceivable to us.
Ultimately, all that remains of the lich slowly and inexorably decomposes beyond recovery. All that remains of the dreaded creature is its skull, the former receptacle of its formidable mind. It is whispered by those who seek to become such an abomination that this fate is not the end. Some believe that the lich is not dead, but has ascended to a higher level of contemplation. Some believe that the creature ascends to explore knowledge that is beyond the trivial, petty, and mortal concerns that the rest of us have. Of this, I cannot be certain.
Despite the whispered rumors, there is a certainty. Once the lich crumbles to dust under the weight of time and decay, it has no interest or use for the world as we know it. Even if it is not entirely dead, it is at the very least, effectively dead.