This build focuses on a strong early and late game for Kassadin.
UPDATE: In light of the recent changes I have made an extensive overhaul of the following sections.
Working in the Team
| Void Stone
| The flat damage reduction granted by this ability gives Kassadin some needed survivability. The attack speed is a nice thought but you will rarely notice it.
| Null Sphere
| Single target nuke/silence that has a short cooldown but a relativley high cost. It's nice to have access to it at lower levels but the mana cost does increase as you level it making it a poor choice to max first. However, it's also a bad idea to neglect leveling it past 1 because it starts at a mediocre 1.142857142... damage per mana (dpm) but gets to a respectable 2.545454... dpm at level 5. It gets a good amount of damage from AP, 0.7 per point. The projectile speed on the actual sphere is slow compared to other spells meaning that people will be able to react to it before it hits (shields, Flash). A positive thing about it however is that allows for some interesting strategies such casting within tower range and then leaving that range before impact.
| Nether Blade
| Mana generation on every attack. Not too great unless you plan to be auto-attacking alot. I usually put a point into this at level 4 so that I can activate it to give myself a needed force pulse charge. The other time you want to use it is if you are given the chance to attack a tower.
The change from percent mana steal to flat really put the nail in the coffin for the passive being at all useful. In ages past, you could whack a mage champion and get a free spell, but now you can get a whole 60 mana (a free nether blade activation! :/ ). However, flat values really shine early game! ...if you were to max this first.
Just pretend (it's not hard) that there is no passive part of this ability.
| Force Pulse
| Although it has a short range and a somewhat strange cooldown method, this spell is one of the most efficient in the game. Spending 80 mana to deal 300 base damage in a cone and slow each target is a great deal, coupled with the fact that it has a great AP ratio (0.8). Max this spell first. Early game the best time to use this is to get some minion kills and hit a champion.
| This is arguably the most versatile spell in the game, being a blink on a 4 second cooldown. It also deals damage which is both a blessing and a curse; if you cast it again within 7 seconds it deals 120 more damage but costs 100 more mana. A blessing when using it to enter combat or chase but a curse when running away. Always be aware of how many stacks you have. Put a point into it whenever you can (as with most ultimates).
Mender's Faith (1)
Reduces the cooldown of your Heal spell by 30 seconds.
Increases your Champion's magic resistance by 6.
Increases your Champion's armor by 6.
Strength of Spirit (3) Requires 4 Points in Defense, 3 Points in Resistance
Increases your champion's health regeneration by 0.3% of your maximum mana.
Evasion (3) Requires 4 Points in Defense
Increases your Champion's dodge chance by 1.5%.
Veteran's Scars (4) Requires 12 Points in Defense
Increases your Champion's health by 60.
Ardor (3) Requires 16 Points in Defense
Increases your Champion's Ability Power and Attack Speed by 2%, and by an
increasing amount summing to an additonal 2% at 18.
Tenacity (1) Requires 20 Points in Defense
Decreases all damage dealt to your Champion by 4%.
Good Hands (1)
Reduces the time spent dead by 3.33%.
Increases your Champion's total health and mana regeneration by 4%
Awareness (4) Requires 4 Points in Utility
Increases the amount of experience your Champion gains by 5%.
Insight (1) Requires 8 Points in Utility
Clarity will now grant the same amount of mana to allies as your champion receives.
I decided to the opposite of what most people do with Kassadin as far as masteries go. I go 0/21/9, using items to enhance his offensive capability instead of masteries.
UPDATE: After trying out other recommended mastery configurations, I find none of them to be as strong early game, and the absolute worst thing that can happen to you is being under-leveled. Defensive masteries coupled with awareness provides a good deal of protection from that.
Marks: Greater Mark of Vitality
Seals: Greater Seal of Vitality
Glyphs: Greater Glyph of Celerity
Quintessence: Greater Quintessence of Fortitude
Static HP runes aren't what they used to be. I decided to try out dynamic HP and I must say that the early game HP loss really isn't as bad as it is with other rune types. I kept the quintessences as static HP however since I didn't want to get too squishy. If you truly wanted to get more EHP from runes then resist and armor would be the way to go, but that's not the whole point of the HP runes. The secondary objective of the runes is to discourage attacks on you. If the enemy sees more chunks in your health bar, they generally won't call you as a primary target. Of course this doesn't work if they have a caster as a linchpin or you are owning already.
I go with Clarity and Heal because it gives you good lane longevity and unpredictability. That extra boost to mana can mean the difference between a riftwalk to safety the following scenario:
The walls surrounding Baron Nashor's lair have never seemed as tall as they are now. I struggle to stabilize the ever increasing chaos of the rift that I will need to traverse to surmount this obstacle. But in my haste, I did not realize how unstable the rifts were getting. Indeed, I would require a massive amount of mana to step through the next rift. As I ready myself for the strenuous task ahead, the sharp pang of terror slips through my mind; I don't have sufficient mana left to cast riftwalk. I swallow my pride, try to suppress my fear and turn to face my aggressor. And indeed, before me stands my sworn enemy, Cho'Gath, the Terror of the Void. The stone I took with me from my venture to the void will save me from the most gruesome fate, but I fear my time has run out. The accursed creature shrieks horribly at me, and the anarchic energies he commands cause my spells to fizzle. He stomps, and I feel the horrible spikes grind against my ribcage. I have failed. I have failed to protect Valoran from the Void. This is not how I imagined it would end...
See the abilities section above for elaboration.
Item Build Order
FIRST TRIP BACK
[item=kage's lucky pick]
SECOND TRIP BACK
Tear of the Goddess
THIRD TRIP BACK
Boots of Speed
LANING IS USUALLY OVER BY NOW.
GAMES USUALLY WON'T LAST THIS LONG, BUT IT'S BEST TO BE PREPARED FOR WHEN THEY DO.
IF A GAME GOES ON THIS LONG YOU WILL NEED THE EXTRA MELEE DAMAGE TO HELP DESTROY TOWERS.
I HAVE SEEN GAMES GO ON THIS LONG RECENTLY. AT THIS POINT PEOPLE USUALLY HAVE LOTS OF HEALTH SO YOU WILL NEED A WAY TO DEAL WITH THEM.
[item=kage's lucky pick]
With the Strength of Spirit mastery, Heal and Clarity you should be set as far as regen goes, which allows you to pick up this item for a nice early game boost to your spells as well as extra gold income.
This item not only holds the record for the most mana at +1400, it also grants the most flat mana regeneration at 25 MP5. As if that wasn't enough, it rewards you for having lots of mana with ability power. I shouldn't need to explain why Kassadin needs a big mana pool (Riftwalk).
There is a reason Mercury's Treads are the most expensive boots. The passive on this impressive pair of shoes is ridiculously good. Sure it will reduce the duration of Time Warp from Zilean or Ashe's Frost Shot, but what isn't readily apparent is the implications of reducing the duration of the debuff. You will take 35% less damage from Teemo's Noxious Trap, Shaco's Two-Shiv Poison, Ryze's Rune Prison, and many more. The magic resistance is just icing on the cake.
Even though you will be getting lots of survivability from masteries/runes you will need more HP at this stage. Many people decide to grab Rod of Ages as their first item to remedy this situation but in my opinion it leaves you too weak to lend much aid to your allies in the early and mid game. The scepter will give you a nice boost in both HP and AP, and the slow effect is a nice bonus (yes, it does work on riftwalk).
After much analysis from a friend of mine, he determined that Void Staff is one of the most efficient (as in cost versus power) items ever. It's unfortunate that I can't get this sooner, but everything else needs to come on time.
Since you usually get off an auto-attack after the three-spell barrage, this essentially gives you another nuke.
As I said before, most games won't last this long but if they do, you will need the HP burning power of deathfire grasp to remain scary (and I assume you are scary at this point since the game has either been going on a LONG time or you are awesomely fed). The cooldown reduction is very nice as well.
Defensive masteries, Clarity and Heal give you good early game staying power.
Archangel's Staff is strong all the way through the game.
Mercury's Treads give an unparalleled amount of survivability.
[item_text=Rylai's Scepter] gives you just enough HP to discourage being focused in team fights.
You end up with lots of AP and mana.
Doesn't get as much HP as Rod of Ages builds.
Armed with Clarity and Heal you have a generous amount of regen. Use Force Pulse for harassment and only use Null Sphere in serious fights. Use your potion and Clarity to stay in your lane until you have 330 gold, at which point feel free to recall, buy [item_text=kage's lucky pick] and return to your lane. Once you have 980 gold or more, recall back to grab Tear of the Goddess (and Boots of Speed if you can afford them). Go ahead and return to your lane but keep in mind that lanes start to become nebulous at this point as we transition into mid-game.
Be there for every team fight, as your silence, slow, and AoE will turn the tide of battle. Only buy when you can easily get back to the front or when you are dead. Preferably the former. Follow the build order. Be extra cautious during the time between Mercury's Treads and Giant's Belt; this is when this build is weakest. After getting the belt, finish off Archangel's Staff as you enter the endgame.
With the staff, you have gained a lot of damage and mana regen. Show your opponents that they should fear you. Enter combat and obliterate with your burst damage.
Working in the team
Don't stay in combat for extended periods of time; that's just not how Kass rolls. The way to properly fight is to enter combat with a rapid, and I mean fucking rapid, sequence of the following spells in order;
This will shred most squishies. After that volley withdraw out of melee range until Riftwalk is off cooldown and then do it all over again (in team fights Force Pulse becomes available very quickly). If the enemy team starts to retreat punish them. You are one of, if not the best chaser in the game. Don't be reckless though! You are also the one of the best runners in the game (second only to that grinning judas) but you aren't invincible. Know your strengths and weaknesses. Keep an eye on your mana. Keep an eye out for melee DPS, your toga provides little protection against the sword.
Kassadin is a lot of fun once you learn to play him and strikes fear into the hearts of casters. I hope you can have as much fun with him as I do.
Curse you Riot for taking Haunting Guise away from me!