A lich is an undead spellcaster of great power, usually, a wizard, but also some clerics choose to take this path.
Liches are generally gaunt and skeletal with withered flesh stretched tight across horribly visible bones but can vary greatly in appearance depending on their age. Some appear as skeletons dressed in regal finery, yet others might appear to be nothing more than lepers. Bright pinpoints of crimson light burn in the empty sockets of those whose eyes have been destroyed or otherwise lost or were so old they had simply rotted away.
Liches often do not have lips or the necessary organs to produce natural speech, but they can project their voice, normally from their mouths, in order to communicate with those that they do not share a telepathic link with.
Because liches have eternal longevity, they often used this time to form schemes that take decades to develop and often plan for every contingency.
The process of achieving lichdom required that the spellcaster construct a powerful magical artifact, a phylactery, in which the lich stored its life essence. As long as this phylactery was unharmed, the lich was immortal and will reassemble if it was ever vanquished.