Howard was influential in the Sword-and-Sorcery genre that gave rise to D&D, but NONE of those are liches. Thugra Khotan is a mage who puts himself to sleep for a couple thousand years because invaders were about to sack his city. Xaltotun is a deceased high priest who is resurrected exactly ONCE by arcane dabblers using a powerful artifact in the hope that he can teach them secrets about the dark arts. Howard describes them as having unnatural traits, but Howard describes ALL wizard-types as unnatural regardless of whether living or not. A better example of a lich in classic fantasy literature IMO is no less than Tolkien's Sauron - who reforms repeatedly after death, whose life force is tied to a specific item (the one ring), and who, yes, is the equivalent of a sorcerer by middle-earth standards. Sauron operates completely off screen (at least in LOTR), which helps to dehumanize him, play up the theme of an alien mind, and maintain the impact of the threat he poses by keeping it unknown and/or ill defined.