Legendary wizards have the option to conduct a ritual of immortality with a chance of being killed in the process. Necromancers can ascend to lichdom with a lower chance of being killed in the process but at the cost of being affected by lich decay that can wither them away if not attended to. Vampirism grants immortality, but if discovered to be a vampire, people will generally be less than impressed with you and (for the time being, at least) known vampirism generally leads to expulsion from societies, though pureblood vampirism does confer a stat bonus. Followers of the Hagraven cults with high piety and meeting certain other criteria may be given the option to become either a briarheart or a hagraven, both of which are immortal. Potions of longevity can extend your life but are nowhere near a substitution for true immortality.