Liches are the most powerful type of Undead in existence. They are the greatest crime against nature. Liches are not created from normal means as most Undead are. Liches are self-created magical Undead, capable of living eternities with no issues as the mortality within them has fled. Liches are seen by the majority of mortals, and even immortals, as terrifying, powerful, destructive, vile beings that one should slay at first sight; or more likely flee from as quickly as possible.
Liches are not naturally occuring. Nature does not produce Liches and even more normal Necromancy cannot create them. Liches are thought, only by heresay, to have been created by those who scorn death as the ultimate fear and forcibly extended their existence by magical and dark means. Today, Liches are scarce and some even thought to no longer exist because of the intense magical prowess required to form one. Many Liches, if they exist at all, tend to be out of the sight of the living, slowly accumulating magic or rotting ever so slowly.
Liches are created by only the most powerful of mages. They are created by the masters of Life and Death and are created from themselves. A Lich cannot be created by someone else through the conventional means. Those who scorn death and forcibly make their bodies continue to last through magic and rituals of the soul and body are formed into a Lich by their own doing. Many powerful mages do not become Liches and will greet Death, but those who fear death or have unfinished business on the mortal plane may force their bodies into Liches. There are a large collection of accounts of powerful, well-experienced mages failing horribly in the attempt to achieve lichhood. It seems that a majority of people cannot turn themselves into Liches, even with decades of experience- leading to a belief that the rituals and potions and ancient magics that allow a Lich are incredibly complex and person to the mage using it.
A Lich does not have their soul on their person. In the ritual to become a Lich, the soul is removed from their own body. With the weight and pressure of a soul no longer threatening the body, the body dies and is forced to exist without a soul by the magic of the user. Though the soul of the mage and the Lich body are not physically connnected any longer, they must remain close. The soul can persist without the body, but inevitably, the Lich cannot be without the soul at least in existance. Consequences of such can cause the Lich to either bend their will to the one who has it or face the death they tried to evade.
A Lich is variated form of Undead. Normal Undead are either mindless or controlled by some other power. Liches are self-controlling and are actually quite intelligent, usually. When the Lich is created, their bodies are just as or more powerful than their normal mortal forms. Achieving greater strength or vitality due to their souls longer being a part of their physical bodies. Also, a Lich's body can be destroyed or bound and the Lich having little to no care at all. It's known that a Lich's body may be eradicated, but reform or heal itself entirely after an unknown amount of time.
Along with, a Lich is capable of casting powerful magic regardless of being Undead, sometimes attaining more magic than what they had when mortal. All Liches gain the benefits and tribulations of Undeath, however usually a Lich has the magical capacities to scorn the costs of Undeath.
Though easily confused, Healers are capable of resurrecting themselves as Undead and are not Liches. Becoming a Lich is an entirely different, more powerful, more unique, more costly process. Though a Lich is an Undead, an Undead is not always a Lich.
When a Lich is formed, a requirement is the removal of the Liches' soul from the body. When done so, the soul does not go to Nirvana but into a prechosen item, known henceforth as a Phylactery. Phylacteries are mysterious and complicated objects. They are where the Liches' soul is hidden and the source of their immortality. While this item is destructable, it is very difficult to do so. Phylacteries are, in some cases, semi-sentient and very magical.