Kassadin has a ridiculous ban rate because he is a very good champion. Null Sphere can shut down an enemy caster by silencing them and that is one of the many reasons why Kassadin is a very annoying champion to deal with. Riftwalk is the other reason that makes Kassadin very hard to deal with because with a 6 second window for Kassadin to be vulnerable, Zhonya's Hourglass can help delay the damage for 2.5 seconds and Flash is a mechanism that can be used to gain some distance as well.

The reason that Greater Mark of Magic Penetration instead of Greater mark of hybrid penetration is because auto attacks are rare for Kassadin. Since Kassadin is a melee champion, aside from csing in lane, not a substantial amount of auto attacks will be used. Most of Kassadin's burst comes from Riftwalk, Null Sphere, and Force Pulse; therefore Greater Mark of Magic Penetration is much better than greater mark of hybrid penetration. The rest of the runes are pretty self explanatory, Greater Seal of Armor so auto attacks don't do too much damage, greater glyph of scaling magic resistance or Greater Glyph of Magic Resist for taking less magic damage, and Greater Quintessence of Ability Power for flat ap damage

For items, it always depends on whether or not you believe you need more mr or armor. With Kassadin, health is not a very important part of his kit in my opinion. The reason that Rod of Ages is not on the list of items to build is because I think that with Zhonya's Hourglass and Seraph's Embrace, there is enough survivability for you to get the job done. I always build Tear of the Goddess instead of Chalice of Harmony because Void Stone reduces ap damage dealt to Kassadin

For skill sequence, to do the most damage, you should use Riftwalk into Null Sphere into Force Pulse, but if it is too dangerous use Null Sphere into Force Pulse then Riftwalk into safety if needed. Nether Blade can be used to charge the stacks for Force Pulse in lane, or used to after Riftwalk in a teamfight to ensure extra damge

pros
- Riftwalk is one of the best tools in the game to escape a sticky situation
- Null Sphere silence does wonders in team fights when properly used
-last hitting under turret is very easy with Nether Blade
- Force Pulse is an aoe slow that does a lot of damage
cons
-when Riftwalk is not available, Kassadin is very vulnerable
-in lane phase, Force Pulse is not very useful because you have to charge it up
- using Riftwalk unnecessarily can cause Kassadin to go out of mana quickly
- Kassadin doesn't have hard cc

Early game, Kassadin's creeping is very dependent on your ability to last hit under turret until Kassadin's Force Pulse is strong enough to clear the wave and push the lane. After level 6, with a little bit more ap, Kassadin can last hit or clear just fine in the lane.

Kassadin can be very useful for invades and ganks because his silence keeps the enemy from flashing over a wall for 1.75 seconds which can be very useful. Since Kassadin's Riftwalk allows for very long gap closes, even if the jungler over extends a little bit, Kassadin can follow up with very little problems.