It’s a well-known fact: the Human race craves for knowledge and power, and to get it they are ready to open doors that should better stay closed. The door of the Afterlife is one of them.
The path that leads a Human to turn into a Lich is long and tortuous, as much as the one leading from Beastman to Abomination and from Demon to Angel. It involves the collection of very rare resources, crafting of powerful artifacts to channel the magic, and Divine quests.
The final ritual of the Lich transformation sees the Human character join the embrace of Death to be reborn as a cadaverous, almost skeletal undead creature, with a whole new set of weaknesses and powers. Some are common to all undeads, such as weakness to fire, increased resistance to cold spells, and the need to feed oneself now gone. Others are more specific, such as a particular affinity for the Necromancy and Abjuration schools of magic.
One final word here. You thought a Lich has an Evil alignment? Just because it’s not technically alive anymore? Bad! Prejudice! A Human turning into a Lich preserves its Karma and Alignment – it’s no different from a regular Human in this regard!