This guide (more of a build really) is basically because I feel like the top rated guide is either outdated or simply lacking (might be it just doesn't fit my playstyle).
This guide is much more emphasized on offense while still giving you massive amounts of HP for survival.

The build in this guide was built through trial and error after first-timing Kassadin and seeing the top guide not fit.

Pros
+ Amazing amounts of damage,
with this build I usually do over 2000 damage in a single combo
And that without the Rift Walk stacked.
+ Global slow with Massive amounts of damage
+ Mid-Game and on, takes on almost any caster.
+ Riftwalk is an amazing gap closer, and a built in flash
+ Riftwalk is invisible in bushes, and can move you through terrain
So be a ninja and Riftwalk through the jungle to get easy kills on the unaware
+ Unkillable if Riftwalk is mastered.

Cons
- Awful early game.
- Squishy.
- Really Hard to Master.
- Riftwalk is extremely mana hungry so must be stacked wisely.

Really standard 21/0/9 AP masteries, nothing special.

Same as masteries, Magic Pen + Mana Regen + AP basically do as my runes, Kassadin is not my main (Love him, but haven't played him enough to call him Main). So rune experiments haven't been done yet.

Void Stone - Sometimes overlooked, Kassadin Takes 15% less magic damage, making him an AP slayer.

Null Sphere - A strong silence, combined with Void Stone can give you the edge when exchanging hits with an AP middler. This is the first skill you have to max.

Nether Blade - I usually max this last, and add the first point only after reaching lvl 7. The reason for that is that Kassadin's early game is bad, so you have to be able to deal as much damage as you can, because without Riftwalk You usually can't make the kill.

Force Pulse - Another Ani-caster. You need to stack it 6 times to use it once, so being against a caster who must use his abilities (And by that helping you recharge it) will again give you the edge against him. This skill massively slows the enemy, while still dealing amazing amounts of damage, making it another chasing or fleeing spell with amazing damage. I max this second.

Riftwalk - Amazing ulti. With it you can escape, close gaps, move undetected through terrain to make the kill, Jump in on unsuspecting champions, dealing massive amounts of damage with your combo, and them being silenced and slowed can't do much to return the damage. The skill stacks damage as you use it consecutively so it's great for going all through the map, then jumping on someone with 10 stacks to almost kill it instantly (although beware of mana hungriness as the mana cost grows as well). A Kassadin that has mastered this skill is Unkillable.

Your Combo is pretty simple.
Until lvl 6 you just hit your opponent with Q to try and scare him away from the minions, denying XP and Gold, but beware not to get out of mana, always try to keep your mana at a minimum of 20%.
Lvl 6 on is where the fun begins, You can Riftwalk on your enemy, Null Sphere them to block any Aggresive skills from being used, Force Pulse them to avoid chasing and simply walking away mostly unharmed while they are either dead or running in terror.
In case of the latter, simply keep chasing, Activating Nether Blade for the additional damage, but also for another stack for Force Pulse.

But you need more than a great combo to get the kill. You can stack Rift Walk through the jungle without your target knowing, only to be jumped upon when It's already too late, killing even Champions that would have killed you otherwise 1v1.

Starting Items:

Boots + 3 Health Potion
Is my favorite start, sacrificing some of your damage output (Which you don't need early because you simply can't kill) for versatility and survivability.

Catalyst the Protector
Goes right after for the additional mana, and the useful Unique, healing you and replenishing your mana upon leveling up, and also prepares you for your Rod of Ages.

Rod of Ages
I usually really hate the use of Rod of Ages for it being time-dependent, but this time there are more Pros than Cons.
The additional health simply keeps you alive.
The additional AP... MOAR APZ!
And the additional mana serves two purposes. Early enlargement of mana pool, and Additional AP from Archangel's Staff.

Tear of the Goddess
Yet again, amazing mana pool and mana regen, granting you more spamability to your skills, also useful to get early for the stacks.
(And let's not forget preparing you for Archangel's Staff).

Sorcerer's Shoes
About time to move a bit faster, Need to chase those pesky Champions.
Also great Magic Pen that combo's well with Abyssal Mask (Details on Abyssal Mask section).

Rabadon's Deathcap
Do I really need to explain? Biggest AP item Ingame, adding 140 AP + 30% increase (calculates after Archangel's Staff's Unique for even more ap).

Abyssal Mask
Gives a nice AP boost, with the benefit of Magic Resistance against AP casters plus Magic Resistance Reduction.
This works well with the Sorcerer's Shoes because after the flat Magic Penetration is calculated, the Magic Resistance Reduction can lower their Magic Resistance below 0 for additional damage.

Archangel's Staff
By now you have probably stacked enough mana and are standing on about 3.2K Mana, granting you 45AP + additional 96AP for your mana. At this point You can single-combo almost anything (besides tanks) into oblivion (Or the void *wink*).

Rylai's Crystal Scepter
Another great Item. It slows enemy units on skill hit, helping in chasing fleeing enemies, mostly further increasing Force Pulse's slow.
And let's not also forget it's Additional Health and Ability Power.

Lich Bane - I usually don't like this item for it to require me to do basic attacks amidst my combo. Nonetheless, it's a more than valid option, giving you additional 600 magic damage, combined with Nether Blade for a devestating effect.

Will of the Ancients - Another great caster item, granting you AP and a team's Spellvamp and AP, helping your teammate casters to deal additional damage.

Deathfire's Grasp - Another choice I don't usually go for because of the need to activate an item, but is still amazingly strong. The Active of this Item deals 25% of the enemy's current health in magic damage devastating enemy tanks.

Void Staff - "Oh wait, what's it doing in both Do Not Get and Situational items?"
The answer for that is simple. Although I'd like to a- Void Staff as much as possible, some rare teams have higher than usual MR meaning you do need that bit of damage.

Banshee's Veil - A waste of an item. the passive can be easily countered by any basic ability, rendering it useless. Better use the slot for damage output.

Void Staff - I usually advise against Void Staff. The Magic Pen is highly overrated in my opinion. It's a percentage, meaning very low benefit on low resistance heroes. Abyssal Mask on the other hand, considering you always stay close combat, has a flat reduction, giving you the better edge most of the time. If you get any benefit taking Void Staff over Abyssal Mask Against a particular champion, just let your teammates handle him and deal with the rest.
To fully emphasized the benefits of Abyssal Over Void, let's do some math:
If an enemy champion has 120MR, boots reduce it to 100, then either Void Staff reduces it further to 60, or Abyssal Mask reduces it to 80, seems like void has the upper hand, but let's not forget carries, which should be your main taget usually have less MR, resulting in a much smaller benefit if at all. Also let's not forget you are not the only one benefiting from the effect of Abyssal, because of the Aura of MR Reduction, and lets also not forget the MR You get against enemy casters.

Before Late-Game, you will probably kill at least two or three times making you a target to be focused, that is your downfall, but also the enemy team's fall.
If you are not focused, combo their carry to insta-void.
In case of focusing (Which should happen eventually), Carefully combo them, then Riftwalking away to safety, letting your team shred them while they hopelessly try to chase you and kill you. They give up? Combo another one of them, giving your team another (usually unneeded by that point) edge in the teamfight.

Last Matches, two of them done with 2 AFK

I signed up and made this guide simply because I haven't seen in the top rated guide a guide that is sufficient enough for me.
What started as a build suggestion turned out to be much more in-depth that I thought.

I hope it helped you banishing Cho'Gath to the void.
Also the usual: Comment before downvoting, giving me a reason why so I can change the guide.

Bye. ;)