A Maldraxxian Lich
House of Rituals
A lich is an undead sorcerer who possesses tremendous magical power. Prior to his transformation into the Lich King, the Warchief of the Horde, Ner'zhul, had under his sway many orcs who controlled various magical forces including warlocks, shamans and death knights created by Gul'dan in the aftermath of the First War. When these powerful individuals were captured along with Ner'zhul by Kil'jaeden and the Burning Legion, the Eredar Lord twisted them into undead abberations and bound their will to that of Ner'zhul, who had been transformed into the Lich King. As a reward for their loyalty, the Lich King gave all his liches the power to control the furious icy elements of the North. Able to control the powers of frost along with their own considerable necromantic magics, liches are some of the most powerful agents of the Scourge.
Liches can come from any race and background. Many have been plucked from the ranks of humanity, offered immortality and power in exchange for their unwavering loyalty and obedience. The archmage Kel'thuzad was one such convert, and several other liches were created after him many of which whom come from the ranks of the Cult of the Damned. Some trolls apparently are also aware of this ritual, as evidenced by the preponderance of liches in Zandalar, as well as the return of Zalazane to the Echo Isles in lich form.
Liches originate from Maldraxxus, the source of all necromancy. Liches are uncommon among the Maldraxxian forces, even among the House of Rituals, who specialize more than any other House in necromantic magic. They are the greatest spellcasters among the Undying Army, utilizing their talents to defend the Shadowlands from invasion. To become a lich, a sorcerer must undergo an extensive and complicated ritual which creates a phylactery, a soul container which stores the lich's eternal soul separate from the body, and creates a new magically generated body which operates independently from the phylactery. This process of removing the soul from the physical form is what gives liches their unusual and skeletal visage.
Servants of the Scourge
At some point, the knowledge to create liches fell into the hands of the Burning Legion. The demon lord Kil'jaeden used this magic to transform Ner'zhul and several of his servants into liches, mentally bound to the will of the Lich King. These liches would act as the hands of the Lich King and worked to spread undeath across Azeroth in service of the Burning Legion's plans for the conquest of the world.
During the reign of the Lich King, liches were often placed as generals within the Scourge army, often acting as the voice of the Lich King himself during times of war. They are power-hungry, manipulative and vicious and they use their considerable abilities not only to serve their master the Lich King but also to undermine and often destroy other liches, often through sabotage or political machinations. They resemble skeletons, but are often surrounded by a blue glow that radiates a constant chill and dress in long flowing robes, often adorned with gems and other precious metals.
A lich may be raised from any mortal with significant magical ability and the willingness to serve the Lich King. While traditionally this translates into necromancers there are some shamans and priests who have become liches loyal to the Scourge throughout recent history.
Not all liches, however, serve the Lich King or the Scourge. During the period of time in which Sylvanas Windrunner broke free of the Lich King's control, many of the liches that were previously loyal to him did as well, with several changing their allegiance from the Scourge to the Burning Legion. This dissension was further enhanced after the death of the Lich King at Icecrown Citadel when almost all intelligent undead were freed from his mental domination and were finally able to act on their own accord. While many have chosen to remain with the Cult of the Damned, others have become independent, working to further their own goals rather than the goals of any unseen master.
Killing a Lich: Phylacteries
In order to become a lich, the mortal undergoing the process is required to bind her soul into a specific object. This object is known as a phylactery and is, at its base form, a soul battery through which a lich can regenerate his physical form should it be destroyed in combat. In order to permanently kill a lich, the lich's physical body must be destroyed, and the phylactery must also be destroyed, for as long as it remains intact and undamaged the lich will always be able to return to plague the land of mortals once more. To that end, a lich is very careful about where his phylactery is located, and they will often go to great lengths to hide it as to ensure that it can never be destroyed, though some seem to carry it on their person and leave it upon the destruction of their body.
If one is to destroy the phylactery, but not destroy the lich's body, its soul will return to the current body. The lich will usually then retreat to a safe area to create a new phylactery.