Lich

A lich, sometimes called a lichnee, was an almost universally evil form of undead spellcaster of great power, usually a wizard, but also possibly a sorcerer, warlock, or cleric. Liches were feared by mortal beings for their malign magic, their intelligence, and their willingness to embrace undeath for a chance to live forever—or rather, to endure forever. Lichdom was arguably the most powerful known form of undeath.
They achieve this from a Phylactery that they have created.