Pros:

• Hard counters nearly all AP mids
• Has extreme mobility and roam potential
• Can 1-shot a carry without pure damage items
• Has enough utility to make comebacks even when drastically behind
• Has Riftwalk , the best ultimate in the game
• Can last hit under tower with Nether Blade
• Is, in my opinion, the most fun assassin in the game

Cons:

• Can be countered hard by some ad mids
• Can be shut down if camped hard
• Is banned in 84.74%(JESUS!) of ranked soloque games as of 12/28

Offence

Mediocre Early game, this can actually screw you over pretty hard, as you're often taking harras without returning it. As you're melee, it can payoff late game, so put a point in if it suits your playstyle

Required Required. This will not only greatly improve your early game, but you will rarely be capping your CDR, so this boost is never wasted.

Good With this mastery you will be able to last hit much more easily. We take it to make our level one easier and for the teir 2 mastery

Mediocre This spell can be nice to set up ganks with and ok in teamfights, but I would stay away from it for the most part.

Required Not much to say here, this mastery is mandatory on all AP carries.

Required If you picked up Butcher , this is a MUST. Gives you great sustain in lane before Catalyst of Aeons.

Mediocre This may seem tempting, but with an in-and-out style of play, you won't get to much from this one. Take only against teams with lots of melee range.

Required Again, not much to say about this. The early game damage increase is substantial, never miss out on this as any AP champion.

Good It may seem odd, but additional points into this actually increase the health at which the bonus applies, not the damage itself. Always put 3 into here when you take it.

Mediocre This, like spell weaving, is a very situation mastery. If you do take spell weaving, I'd likely leave this one out anyways, as your base attack damage is mostly irrelevant.

Required While mana and health are important, at the end of the day we are just as reliant on AP for damage as any other mage. Don't pass this up for anything.

Needs Testing This mastery is very hard to rate. Usually I would pass it up, as the 5% return isn't great, but as someone who needs to get in, kill someone, then get out, this mastery seems decent on Kassadin. Requires further field testing, go with it if you want to, and tell me how it goes.

Mandatory The additional penetration makes up for the lack of raw AP items early on, don't leave this out of any of your pages.

Good This mastery is good on all Ap champions but especially Kassadin. It works very well with your Nether Blade and Void Stone, but is not as necessary as some of the other masteries.

Defense

Required This is a must when going into the defense tree. Your biggest weakness early game is ranged or even melee auto attacks, and this can help a ton.

Good This is the other mastery to take when going into defense. It gives you nice sustain early game, something Kassadin desperately needs.

Good This further reduces the damage you take early game. Definitely grab it if you put points into Block .

Required The main reason you go into the defense tree, this mastery can mean the difference between life or death in an early trade. An absolute must when going into defense.

Required This is extremely useful lategame when we have a Rod of Ages. This will bee more than worth it with just a little health, you might want to go into the defense tree just for this.

Utility

Good Pretty self explanitory, this can be nice to get you out of sticky situations, but don't sacrifice anything crucial for it.

Good Movement speed is one of the best stats in the game, but you don't get a ton from this, so it's only just a good mastery.

Required If you aren't putting 9 into defense, pick this up immediately. The mana regen is useful even when you have Catalyst the Protector and Tear of the Goddess.

Mediocre Meh. It's a bit useful for when you want to ward, but other than that you can probably be fine skipping this.

Required With teleport and flash, the total time reduction on your summoners comes to a full minute of cooldown reduction. Need I say more?

Good This can be a very effective mastery late game, but as it only really helps when you're nearly complete with your build, passing this up is an option.

Good Like Fleet of Foot, this mastery works very well with Kassadin but simply doesn't have huge numbers to back it up. We will always take this though, as it goes into Culinary Master

Mediocre If you are allergic to biscuits, grab this to increase the duration of any buffs you are given or steal.

Required There is very little reason not to grab this mastery. You always start with at least one health potion, and this will allow extra mana sustain in the early game.

All other masteries simply do not work well with Kassadin and should not be considered.