The secrets of lichdom have been sought by many, and with good reason. Immortality and invulnerability from harm is tempting for any mage who fears the ultimate finality that death will bring to their studies. Even if their body is wholly destroyed, the phylactery in which they keep their soul-essence will restore them to physical form. Many books have been written on the attainment of the lich state: the Liber Animus, the Tome of Acerak and the Grey-Skin Grimoire to name a few.
Becoming a Lich
Although the methods and rituals detailed in various lich grimoires vary, there are certain things which remain constant. The first of these is the phylactery: a magical container of the highest value to which the life-essence of the lich is bound. Even if their physical body is destroyed, the lich will reform in the vicinity of their phylactery within 1 to 10 days.
The next constant is magic: only mages of great power can become liches. Although the specific methods of attaining lichdom may vary, they always involve the exertion of massive magical energy and powerful workings. A combination of immense power, vast resources and great reserves of skill and tenacity are required to bring lichdom to fruition.
The final constant is evil: though there may be good liches out there, the act of becoming a lich is inherently and irrevocably evil. The incredible power that guarantees eternal undeath and invulnerability to a lich does not come cheap; their phylacteries require a power source. The ritual of lichdom always includes at least one sacrifice of a sentient being. In some cases, the soul of a victim may be placed into the phylactery first, and consumed. In others, their living blood may be added to a potion which is consumed by the would-be lich. But there is always a sacrifice.
The power of a lich is based upon the sacrifice of souls; without it, he would be unable to sustain his form. Even once he has attained lichdom, a lich must continue to supply his phylactery with souls to consume if he wishes to retain it. This does not need to be done frequently; even a lesser soul might sustain a lich for a decade, and powerful souls can last for centuries. It does not need to be done constantly, either; most liches build up a reserve of souls such that they may comfortably survive for hundreds of years.
A lich has many options to sustain themselves. Many, unwilling to divert themselves from their studies with the hassle of seeking out souls, will barter with Night Hags from the Lower Planes for larvae. Others may conjure the souls of the unconsecrated dead, and devour them. Still more may take a more active route - either finding victims themselves, or installing themselves at the head of a sacrifical cult.
Destroying a Lich
A lich may neglect his phylactery, either though apathy or necessity, causing it to crumble and be destroyed. A phylactery may similarly be destroyed by accident or sabotage. When this happens, the lich's soul will return to their body. Not only are they vulnerable to permanent destruction in this state, but they are no longer protected from the effects of time. Without the phylactery to sustain it, their soul will begin to consume their physical form - causing them to decay as nature intended.
In some cases, a lich who has lost their phylactery will decay completely before they are destroyed by violence. This most often happens when a lich tires of undeath and chooses to neglect their phylactery and decay into nothing, dwelling in a sort of morose coma. Eventually, the negative energy that ties their soul to the Material will consume their body completely, leaving behind only a skull and pile of dust. When the body is no more, their very soul itself will be consumed. The result is a demilich; a faint echo of the lich that once was, with no physical form or personality. They will rise if disturbed, kill, then return to their rest; for the intelligence that once animated them has departed.