There once was a well-loved prince who desperately sought to save his kingdom from a scourge of undead monsters. His resolve lead him step by step down a path which lead to horrible acts of murder in the name of protecting his people. As the war raged on, his quest drew him closer towards a hidden evil power that was controlling the undead army. His resolve to do what was necessary, combined with the fear growing inside himself, slowly weakened his grip on his own sanity. The prince sought a magical sword he thought would give him the power he needed to win the war. In a deciding moment, he reached for it. But the sword was cursed and created a direct link between the prince and the powerful force of evil ruling from afar. Most of his soul was stripped away as he fell prey to the evil power. His people watched in disbelief as their beloved prince lead the undead scourge against his own kingdom, killing his own father and those he had sworn to protect with his life. In time his soul diminished further and further, and his obsession for power grew stronger under the influence of the cursed blade. The prince went on to merge his spirit with the force of evil that had corrupted him, becoming one of the most powerful forces of evil to ever walk the land. He became the Lich King.
A lich is an extremely talented wizard who decides to pursue immortality at a terrible cost. The wizard seeks forbidden knowledge, breaks taboos, and commits horrible acts to perform a dark ritual that will let them live forever. After the transformation, they appear as gaunt, undead humanoids with burning eyes that show not a single glimmer of human concern. They become necromancers that can summon legions of undead monsters who obey them completely.
The process to become a lich is a well-guarded secret. Through my research, I have learned some of the specifics and sources but I believe, and many would agree, that the knowledge is best kept hidden. What I can say is that the process involves the creation of a magical container called a phylactery. The soul of the wizard is trapped within the phylactery and, if anything destroys their physical form, they can regenerate themselves beside that container. The nature and appearance of a phylactery varies considerably. In some stories they have even been called by other names, such as "Horcrux". What is consistent is that the lich will defend the vessel holding their soul at all costs. The location will be a well-guarded secret and the artifact itself will be protected by powerful magic. The phylactery is always incredibly hard to destroy and you will only succeed with the help of special knowledge about the lich and a powerful magic weapon of your own.
The threat and resources a lich can bring to bear is impossible to overstate. Consider the facts that must come together to create such an evil creature. Firstly, the wizard must possess incredible magic strength to master the keys to immortality. Second, they must lack any moral scruple in order to break the taboos of undeath and commit the acts of murder required to complete their dark transformation. Finally, once they sever the connection to their soul, they become immortal allowing them to enact plans that can span centuries. As time marches on, the lich remains isolated and grows ever more meticulous in planning but ever more mentally unstable. This combination of motivations and the power that produce a lich means that their impact on peoples lives is widespread and never good. Not many would escape the wrath of a lich who had seen one of their plans come to fruition.
There are only two rays of hope when facing the threat of a lich. The first is that they are fundamentally selfish. It is likely that they have no real interest in worldly matters unless the world somehow interferes with them directly. However, over a potentially endless timespan, and considering the inevitable loss of their sanity, this hope remains slim at best. The final hope is the knowledge that they can be destroyed if you find their phylactery and have the courage to attack it. The quest will be nearly impossible but it has been done before. Thos who have done it are counted amongst our greatest heroes.