I think Kassadin is a pretty cool guy. Eh riftwalks around the map and doesnt afraid of anything.

Pretty standard runes, except for the quints. HP regen is extremely useful because Kassadin's lane phase is awful; these in addition to the Doran's shield will allow you to sustain through auto/spell harass.

The % magic pen mastery is pretty necessary on AP carries so you can do damage to people with mr.

The 11 points in defense give you hp/armor/regen to survive in lane pre 6 and have extra hp later in the game.

The 10 points in utility gives you the buff duration, which is absolutely necessary on Kassadin because you can do almost nothing without a blue because of his mana costs. The point in the biscuit is to give you an extra bit of sustain in lane.

Kassadin is one of the few heroes in the game that actually needs a large mana pool, this is due to the fact that his riftwalk has a stacking mana cost, so the items you build on him should give you that.

Dorans shield start is to give you defense against ranged auto attacks: Kassadin's worst enemy, as well as sustain and extra hp against all ins or ganks.

Tear and catalyst/RoA will give you a fairly large mana pool which will allow you to stack rifts for damage or rift around the map to gank.

Homeguard boots are amazing on Kassadin. At the point in the game where you have tear, roa, and homegaurds you will be a ganking machinegun. With blue and the homeguard ms bonus you can travel the map extremely quickly while stacking riftwalks for damage, and once you get a gank off you can just base and then do it again instanly because homeguards will refresh your mana/health pool.

AA and Deathcap give kassadin a large amount of AP which is obviously pretty good on AP carries.

Void staff is necessary so you can deal damage to people who bought MR.

Kassadin will eventually need to be tanky because you will be using riftwalks for damage, which can put you in a very dangerous position. Buying Gauntlet or Banshee's will give you tank stats as well as mana, and mana is pretty much damage on kassadin. You should purchase one or the other depending on the damage type that threatens you most on the other team.

Most of the time you will max Q for the silence duration and trading in lane. One point in W helps cs under tower, which you will be doing a lot because kassadin is trash in lane, as well as giving you mana sustain. W can also be useful for damage if you ever get in range to use it. Max E second because it does sick damage. You should always skill your ult when possible because its a flash on a very low cooldown, and it also does pretty good damage.

There are certain situations where putting extra points in E or W can be very useful. In lane against people who constantly spam abilities extra points in E can be great because its on a lower cooldown than q and can help you counter-push.

In lanes against melee heroes that will try to auto attack battle you, extra points in W can help win auto attack trades/all ins. Kassadin will win in a straight up melee battle against a lot of people because of his high base ad and high damage on his w; his passive can also give insane amounts of attack speed if the enemy deals magic damage to you.

Kassadins skills arent very complicated. The only thing you need to be aware of is the amount of E charges you have and the number of stacks of riftwalk you have.

Kassadin is a pretty fun hero, but if you pick him be prepared to not do so well in lane. This build will help you survive lane and give you all the tools you need to gank everyone and make the enemy team mad.