Immortality

Sometimes, a mortal creature will become obsessed with immortality. At this point, they will do anything, including defiling the temple of the deity they have worshiped, receiving a curse in the process and the coveted immortality.

At other points, they will start worshiping gods that can give them what they seek (generally a death or dark-themed one) and that god will send a slab to the world containing the secrets of life and death that will arrive in the opportune place for said creature to take it and learn its secrets, thus becoming a necromancer.