This guide is my Kassadin build for my own reference. I may expand on it someday, but you'll probably notice it appears less offensive than most builds. This is because Kassadin is inherently lethal in the early/mid game and his output is largely limited by his mana pool. Working to alleviate this early with a catalyst will make you more survivable AND more offensive in lane than just about any other item choice. Later you'll be playing for massive rift stacks, and so we take a tear next.

Once you've sorted out your mana pool, from this point on you can start to build as a normal caster. Upgrade to your sorc shoes first and get a blasting wand or a rod as soon as you can afford it. Aim for a Rabadon's first, AA after. If the physical damage on you starts to get dangerous, pick up a glacial shroud. This can be done at any point, but should be done after the AA anyway; we're building into the Frozen Heart for the mana and the massive CD reduction, the aura and armour will help you in your weakest area which is surviving the AD carry. If the CC's getting too heavy for your cleanse to handle at any point you can build into the Banshee's, if not then you could consider selling the catalyst and getting some more AP; I'd normally choose an abyssal scepter as I like to have a bit of armour and a bit of magic resist. The Lich Bane is something of a filler item; it's good to have on Kassadin but you could equally replace it with an hour glass if the enemy team is very physical-damage-heavy, the abyssal scepter if the opposite is true, or if you're doing well maybe it's time for another Rabadon's.

EARLY GAME

Take mid. Until level 6 you have one goal, and one goal only. Farm to 925 gold. This will be hard; melee hitting is risky, don't be afraid to use your null sphere or force pulse just to drop creeps. If you want to try getting melee last hits one technique is to move in to null sphere your opponent, back off for a fraction of a second to avoid retaliation whilst the sphere gets to them, then move in for the last hit whilst they're silenced. Hard to do and doesn't work with everyone (it's not going to happen with Orianna or Syndra) but it's a better use of the mana. If you're doing this, get W level 2 then return to the skill order.

If they get aggressive before you hit 6, your life will be hard. Don't be tempted to duo lane, Kassadin's farm is so bad that competing for last hits will only make the situation worse and almost guarantee you won't hit your stride in time to become worth anything late game. In mid, once you hit 6 everything will change. You won't become a god but this is the situation:

You've spent the last 5 levels playing cagey and looking scared for last hits, your opponent is probably over confident.

You've got your catalyst and you're incredibly tanky for an early game caster with your skill and rune choices, but your damage output is still large.

Start harassing them with your null sphere whilst farming. When they're around half health, move in to null sphere them, they probably won't even back off since they've "learned" you're not a threat over the last 5 levels. Often, they'll even come to get you. You're going to rift onto them, try to do it in time to rift OVER whatever they throw at you if it's a skill shot. You'll land as they're silenced, force pulse, ignite, melee with W if you have it. Congratulations you just got your first kill.

MID GAME

You're at the point when laning is going to come to an end soon but teamfights haven't yet begun. You need to take advantage of this as best you can. If your lane is half-pushed or less to your turret then you can leave it in search of ganks in the other two lanes. If the enemy are half pushed to your turret or more there's a good chance you can kill them, use this as a metric to decide when to gank. There's no magic here; null sphere, rift, pulse, melee W, rift, keep going until dead. You're pretty tanky, you can tower dive in a limited fashion, just be sure your rift will be off CD in time for you to get out. Keep this up as long as you can, your goal is to snowball out of control before the teamfights start. It'll be harder for you to play the assassin role once they start working together, but all is not lost.

LATE GAME

Whether you've snowballed or not you'll play this bit the same way, and although this is your strongest phase you'll probably contribute the least to teamfights. You need to be near your team at all times but not actually with them; in this sense your team will often need to initiate 4v5 so you have to even those odds ASAP as soon as your bruisers are in there. Rift onto whichever carry you kill the fastest, preferably the AD, kill them quick, get out. You'll probably burn your cleanse on the "getting out" bit, and you'll certainly be glad you got a Banshee's veil. Survived with enough mana and health to do it again? Sure you did, you're incredibly tough for a caster, so take out the next squishiest guy. If there's only bruisers left, you may not even have to rift out. Dance around the fight with rift walk to keep your stacks up and do more killing. Here's some more general tips for late game:

• Build a bunch of rift stacks before rifting in to a fight to make it hurt more
• If there's no fighting going on, push a turret; your riftwalks make you too mobile to stop and your mana and CDR mean you'll always be able to rift
• Never rift into the middle of their team if they're not already engaged; you may be able to take out the carry before they kill you but they will kill you
• If you haven't decided on your last item slot yet, pick up wards for dragon and baron, you can go set them quicker and safer than anyone else