Lich is the name given to souls with enough determination to resist the pull of the Ethereal Veil and return to their body after suffering a fatal injury. Lich are not considered undead; they are fully living beings. They require sustenance and can die of old age like a normal human. But their spirits are too stubborn to be torn from life before they're ready, so they return to their body as long as there is a body to return to.
While insubstantial, Liches will often infuse their corpse with Effusia, beginning an expedited healing process. Once their physical body regains the ability to perform vital functions, the Lich's spirit is free to inhabit the body once again.