At first, Erik had too many other concerns to spare much thought for his son in a cell. Pietro's kidnapping had been entirely an accident (the meddling fool, it served him right). There was no dastardly plan as there had been countless times before. Magneto's visits to his son's cell were simply for his own amusement until the accident began to present itself in his mind as an opportunity.
Affairs on Asteroid M were not going as smoothly as Erik might have hoped. People were understandably restless and, of course, there was dissent among those who had been forcibly taken on Earth. Although some were amenable to the idea of a utopian mutant planet, many did not appreciate having been brought to Genosha against their will. It would take time to sway them to his vision, Erik was well aware. In the meantime, there were rebels amongst the Genoshans, and a revolution would be most inconvenient on this relatively small rock hurtling through space. Consequently, Magneto's Brotherhood was forced into a policing role which even he had to admit they were not remotely suited for. All of these problems could be solved once they found a suitable planet to call home, but until that time Erik would use any and all tools at his disposal. That included his son.
In Erik's estimation, Pietro was a weak-willed disappointment, but objectively his mutant abilities were advantageous when properly wielded. With the right sort of manipulation, Magneto thought Pietro would prove an effective weapon in his arsenal. Breaking him would be easy enough, but winning him over would prove a problem. There was little love lost between them, and so appealing to his son's filiality would be pointless. That bridge was burned long past. He would have to find some other approach to bring Pietro to his side.
This Erik mulled over in the months after Pietro was moved to solitary, observing with interest as the isolation and incarceration broke Pietro down. Erik helped the process along with dark hints about the fate of humanity and aloofness when questioned. He wanted to push Pietro to a shattering point so that he could build him back up again into something more useful. An idea of how to accomplish it struck him one day, after addressing the masses of Asteroid M with a speech meant to placate them, all full of grandiose intensity and utopian ideals. Ideals that his son had once espoused in theory, if not quite in practice. Magneto's mutant dogma was one of few things they agreed on. Take away all that mattered in Pietro's life, and Erik would have more to offer him; faith in a worthwhile cause.
"Hello, son," he greeted as he approached Pietro's cell. Time to really gauge the damage, begin the assault. Now here was a nice dastardly plan to get on with. "I have something for you."