There is a huge disconnect between Kassadin's gameplay and fantasy
Fantasy, Lovecraftian looking void guy. Originally a magi who got a glimpse of the void. Not he freed his mind from it's grasp and uses his magic tainted by void combined with a blade and anti-magic powers to fight the void and stop it from taking over.
Gameplay, now originally he had a kit aimed towards this fantasy. Mana stealing, armor pen, AS from magic damage, a voidic silence to stop spells, a slow powered by consumed magic near him, and a step through the void to stay on his target. But it was a flawed, and overpowered design.
Most of it was that his scalings made him oneshot everyone late game instantly popping over a wall. Part of it was high base damage on his point and click silence making him able to trade strong in mid without retaliation.
So he was gutted, gutted, gutted, gutted. And turned from a mobile, mage, anti-mage bruiser. To another AP assassin nobody cares about because they could just play LeBlanc, Kat, Fizz or Akali instead. Who are all better designs that better fit their fantasy(even if OP or UP at this time)
I intend to suggest a specific Kassadin rework at a later time. But for the moment. I just wanted to mention that while it is good Kass isn't 90% p/b anymore. He also isn't Kassadin anymore. And his gameplay just feels bland and disconnected.
This post in no way pretends his original design was good/healthy, and he is well balanced now as a generic AP assassin. But he is also not worth picking as a character fantasy anymore for so many who loved him... at least 3 people I know that is.
)