Where Acts leaves Simon as a minor, unresolved figure, later Christian writers built him into something much larger. Irenaeus of Lyons, writing around 180, names Simon the fountainhead of all heresy, the first man to claim that his own teaching, not the apostles', carried the true hidden knowledge of God, and credits him with founding a school later heresiologists called the Simonians. Irenaeus reports that Simon claimed to be a manifestation of divine power and traveled with a woman named Helen, whom his followers were taught to regard as the first thought of God fallen into matter and rescued through him, a doctrine that made Simon himself the object of redemption rather than its agent.
The apocryphal Acts of Peter goes further, staging a public contest between Simon and Peter in Rome: Simon attempts to fly by demonic power in front of the emperor and the crowd, and Peter's prayer causes him to fall to his death. No canonical text records any of this, and the story functions as a legend about the triumph of apostolic authority over rival claimants rather than a historical report.
Modern scholarship treats the whole edifice with caution. How much of "Simon Magus the Gnostic founder" reflects an actual historical teacher, and how much is heresiological construction, in which later polemicists needed an originating villain and folded diverse rival teachings backward onto the one named rival that scripture had already condemned, is a live question rather than a settled one. The Simon of Acts 8 is a magician who tried to buy spiritual power and was refused; the Simon of Irenaeus and the later legend is something closer to an archetype, built to explain everything the mainstream church wanted set outside itself.