Discussion and Questions Welcomed!

Hello, I'm a season 4 Gold Kassadin main, this is my first guide.
I started playing Kass when I was level 10 back in season 3 and have never looked back, over 500 games in normals easily, probably around 70+ In ranked. I've spent a lot of time testing builds on old Kassadin and New Kassadin and wanted to share what I know.

(Edit: Notes have been expanded to early game, mid game and late game itemisation subheadings. Edit 2: Updated champion matchups removed fizz, updated akali and veigar

Kassadin is a Mobile AP Melee Assassin who excels at farming under tower, Roaming, and Diving the backline in team fights, he has large burst end game and outscales a large amount of other AP mid laners, you should always be at a fight in bot or top lane before the enemy mid laner and your ult is a huge part of that.

There are two main builds to Kassadin, this is by far the fader of the two, RoA is the first key item, provides good health, mana and guarantees scaling into late game, the opposing build to this is the full AP burst mage build, which I would only reccomend if you are strong on Kassadin, very aware of your positioning, andover all are very comfortable on the champion. This build provides health and tankiness as well as good damage and burst, especially in sustained fights where you can get more than one round of spells off.

The opposing build gives less health and mana in tradeoff for being able to 100-0 a carry instantly, using zhonya's and then hoping your team follows up, you have time to R out of the fight after zhonya's (or flash), or trade 1 for 1, Kassadin is a slippery champion but his jump has high mana cost if you spam it and a decent cooldown with no CDR if you choose to go full AP.
For example the burst mage build goes Zhonyas, Rabadons, Sorc Boots, Void Staff, Archangels/Seraphs and Lich Bane, this build is very fun but also very risky, the build this guide is based aroud is by far the "safer" kassadin build and the most recommended.

Pros/Cons for Kassadin as a champion

Pros: Mobility post 6
Good Poke/Sustain against AP poke
Great Roamer
Bursty
Fairly Low CD on abilities
Good at chasing down targets
Can interrupt channels/Effective Anti-Mage

Cons: Very gankable pre 6
Bad against AD Mid matchups
Very Item/Farm Dependent
Takes a long time to scale up
Requires decent mana pool to maintain mobility

Kassadin benefits large amounts from AP and scaling AP blues as he is a high scaling burst mage.
Armour is 100% always an option because he has low base armour and has nothing to negate AD damage.

In high poke matchups its up to you whether to take MR or not, your passive reduces a lot of magic damage incoming and your Q shield provies a decent amount of protection against poke so 9/10 times I take the scaling AP to do more damage, however Kassadin does not gain MR per level so you have to keep that in mind. Certain matchups (see matchups section) will greatly benefit from the MR as well.

Quints are AP and Reds are Magic Pen, no split pen because melee even with W its not worth it long term, Standard stuff.

Brief overview on the masteries, my mastery page has the MS to avoid skillshot based poke as much as possible, 21/6/3 is my preferred overall as since you're building health and starting flask almost always you have the sustain to not need the extra 12 health + 3% max health from veteran scars/juggernaut in defence. This is personal preference mostly due to avoiding ganks pre 6 slightly easier, you can easily run 21/9/0 or 21/6/3 with the 3 in utility in the mana regen if you feel the need.

Summoner Spells are entirely circumstantial
Ignite: ignite is a standard take on a solo mid/top laner, gives strong lane pressure and kill potential, always reccomended.

Teleport: Xpeke is that you? No? Then teleport probably isn't the best choice for you, it's a good summoner in certain situations though such as: you aren't going to aggress early, you REMEMBER to use it, OR you're against a high harrass opponent and want to be able to back and teleport back into lane, very versatile spell, use at your own discresion.

Flash: Just do it.

Barrier: Leblanc, Fizz, Zed, high burst champions that counter you and have a high chance of going all in on you pre 6, barrier is perfect for this, it won't help you kill them but it'll stop them from killing you untill you can outscale them.

Heal: Similar to barrier but if you're ignited (which you probably will be in an all in) then its a lot less effective, low reccomendation.

Maxing Q first is by far the most common skill order for playing Kassadin, however there are rare matchups against champions with low cdr high spam spells (Riven etc) where maxing E CAN be done, however Q provides better more reliable and consistent poke. EDIT: Previously in this guide I said you should max E second for damage, however with the new ult changes that is no longer the case, you max R as soon as possible for maximum jumps at all times.

Farming with Kassadin is fairly easy aside from the fact that you are melee and you are very susceptible to poke and harrass when farming, as well as you have low waveclear which can be an issue against champions who constant push. However Kassadin thrives at farming under tower, high attack damage per level scaling as well as his W which provides extra on hit damage, and is an auto attack reset makes farming under tower much easier, Q as a targeted ability as well makes for easy farming however semi long cooldown at early levels and mana cost mean you shouldn't spam farm with Q (you aren't veigar). E into R post 6 can be used to clear the back line of a wave.

Flask 3 pots:
This is the standard safe build, negates getting bullied out early, there's almost never a scenario you wouldn't start this even if you're following a different build path for end game.

Cloth 5 pots is for hard counter AD assassin matchups (Talon/Zed/Yasuo)

Dorans Ring 2 pots great for early poke, take in lanes where you will take very minimal responding poke, or very weak poke (katarina is a good example)

(Added by Request) Tear Rush Sapphire crystal and 2 pots, only take in very low harrass lanes, other option is fairie charm 5 pots and a ward if you are very concerned about early gank and have high harrass lane but still want tear first (not sure why you would do that but on the off chance)

First and Second Back Items, usually 5-12 minute mark.
Rod Of Ages
Once again the ultimate safe farming item, you ramp up slow but this helps you stay in lane for as long as possible along with flask, blasting wand before catalyst if you aren't against high poke/want to be the one poking.

Tear
is optional in that you can get it if you are really spamming but otherwise I don't think you really need it, if you've got the spare gold its probably better to get the parts to Seekers (especially if you're against an AD) (EDIT: Tear route is now much better due to the jump cd and max mana gain being extremely viable on kassadin now, I still dont think you NEED it, but it definitely helps.)

Seekers Armguard
Zhonyas is great to rush almost always on Kassadin, if against an AD you may even want this before you finish RoA, eg. Catalyst into Seekers into RoA.

This is the 10-20 minute mark.
RoA because Health, Mana, AP, and it scales up with time just like you.

Tear Route If you want to go tear route then you probably don't want to upgrade until you're past 400-500 stacks which you probably wont be early-mid, so hold off on seraphs.

Zhonyas is by far the best choice (after RoA) especially against AD mid laners.

Mejai's is an option if you are fed and want to snowball the whole game out and can do so (please practice a lot before trying this though it can lose you a lot of gold).

Boots Choice is up to you, Merc and Sorc are usually the only options, you'll have Seekers for armour (and armour runes). CDR boots are also an option if you feel the need.

20-30 minute mark, you're at team fights where you need a bit more damage.

Here's where you can diversify a bit, you're fed and they're stacking MR? Void Staff all the way. You're fed and they aren't stacking MR? Rabadons, Lich Bane, Morellonomicon if you want something a bit more CDR based. Frozen Gauntlet is another option that I personally don't find as enjoyable but if your team is ahead and you lack CC I can see a good use for it. If you went tear route this is where you would be upgrading into Seraph's.

Defensive
Whenever you feel you need some defence you have this area. Defensive Items are a bit different, with RoA you've got plenty of health so raw health isn't needed too much, with the benefit of having 40% CDR Frozen Heart can be a great pickup if no one else has one, otherwise standard Banshee's/Guardian Angel to survive a catch or full combo burst when you dive in. QSS always viable against certain compositions (Warwick, Malzahar, Zed or heavy CC teams)

You've made your decisions along the way, there is no perfect 6 item build for every single game, I like this one for the CDR frozen heart gives (especially now with ult late game on 2 second CD), however its fully interchangeable with banshees, spirits, QSS and GA

Roam, just do it, because you can move further in less time than every champion that isn't Sion, Rek'sai or Twisted Fate.

In team fights you want to be looking for a flank to dive the backline after your teams main engage has gone through, try aiming the enemy mid laner for a simple target and their ADC if you think you can take an auto or two from them without worrying, Zhonyas is very important to gain the backline's attention and then zhonyas to provide your team with an easier fight, usually you can get good damage down or chase someone out of a fight, be sure to hold Q if there is someone with a scary channel on their team to impact the fight as much as possible.

Spamming R will cost you a large amount of mana, be careful when doing this to ensure you can still go in/get out when the time comes

When running away on low mana, you can stand in the edge of a bush and use W on the first enemy to enter to gain 20-40% (depending on level) of your maximum mana back, this can mean an extra R away in some cases and can save you from dying quite a lot.

Your Q interrupts channels and is a huge part of why Kassadin is an anti-mage, Twisted Fate's second part of his ult, Katarina's ult, Fiddlesticks Drain (and his ult charge), Xerath Q etc etc can all be stopped by Kassadin's Q ability, keep this in mind when facing champions with channel abilities as it can mean the difference between winning and losing a lane if you use it correctly.