In the times of the Askani, the fabled Twelve were a group of powerful mutants who could have potentially stopped the rise of Apocalypse, but for some reason failed. Information on the Twelve also existed in the modern era, spread by Destiny's diaries and the time-travelling Askani sister Sanctity, who inputed their identities in the Sentinel Master Mold. Rumors and confusion ran rampant about the Twelve's true purpose, and even their identities.
In the end, the Twelve were revealed to be a sham. Apocalypse deliberately started the legends of the Twelve in order to gather a special set of mutants. These mutants acted as living components to the very machine which allowed his ascension in the first place.