This Kassadin build features an extremely passive early game in order to completely pop-off mid and late game. From quick tear stacking to sustaining rune pages, the guide will teach you how to become a late game Kassadin monster.

Pros:
Amazing mid-late game with hyper scaling
Great mobility
Low cool downs
Hard to really screw up on
Dominates most match-ups mid-late game
Still gets through early game with Fleet Footwork

Cons:
Terrible early game (although helped by Fleet Footwork)
Can be bullied early game to prevent farming
Weak into hard ad match ups, especially Talon
Can run out of mana fast without preservation and careful consumption

Take it. Nothing much to say here.

My favorite secondary summoner for Kassadin. Being able to buy a Tear as early as possible and only miss 1 or 2 minions worth of exp and gold is amazing. Also, you don't miss out on much like Barrier with your passive early and you don't need ignite because you will only be fighting mid-late game in which your stacking will do enough damage without ignite.

My second favorite summoner. If you are playing against a hard match up mid lane with hard all-ins like or , or a difficult poke lane where you might be all-inned at low hp like or then Barrier can prevent easy deaths. Barrier can also be replaced with , but considering most mid laners will take ignite, it will be less useful, and the lower cooldown and more damage protection makes Barrier a better choice.

Not a terrible choice, but with this build, you won't need to fight early much, and mid-late game you won't need it as much as teleport is useful early.

There are several keystone runes that you can take, but here are some suggestions from "don't take this one" at the top to my favorite at the bottom.
I don't use Unsealed Spellbook on any champion, so I don't have much experience with it; however, it can be useful when you take Teleport so that you can back for tear and get back to lane and still match an ignite or barrier.
Phase Rush really isn't that bad; it offers safe short trades with an easy escape, but the other options nudge this one down.
Electrocute is a good contender for the second place spot of runes you can take; the damage it offers late game is great, but early game you suffer a lot, hindering your ability to actually getto late game. In general, it's a pretty solid page
My favorite keystone on Kassadin. Fleet Footwork offers amazing sustain in the early game, and it can help you get through hard match ups, too. Also, it's not just useful in early game, the rune can help you catch up to an enemy (although you won't need much of that). Also, the Precision tree offers some pretty useful runes.

Those are the main keystones that you may (or may not) want to take. Next I'll talk about runes that you can take under the Fleet Footwork rune page.
I like Presence of Mind because after a long chase of using several rift walks, you will probably be low on mana. When you kill (or if you kill) your target, you'll be granted a pretty good amount of mana. Refreshing some cooldown on your Rift Walk is cool, too, but it really doesn't matter on a 6/3/1.5 second cooldown.
Legend: Tenacity isn't a real choice that you have to make, you don't need attack speed nor do you need lifesteal. This rune is pretty good though for Tenacity because it lets you riftwalk out of CC quicker.
Coup de Grace is a pretty good rune in general, better than Cut Down or Last Stand. Although there is a lack of decision here, too, the rune is pretty solid

Now, here, we have several options for a secondary page. My favorite is Sorcery, but there are other options like Inspiration or Domination. You can take Resolve, but it doesn't compare to your other options.

Here are my favorite options for Sorcery in each line
Manaflow Band offers good mana sustain in lane, and it's even better mid-late game when you are regenerating your missing mana. This is one of my favorites runes to take.
Transcendence is my other favorite option in the Sorcery tree just because of the early CDR at level 10, but late game it doesn't matter since you will get 40% CDR with your build.
I like Gathering Storm simply because it just offers damage late game. Since this build is all about stacking and scaling, Gathering Storm is a solid option.
Not a terrible option, extra movement speed is not a bad option for Kassadin, but he is already mobile, so it doesn't matter too much.
Don't even try. Don't even think about it. I’ve seen too many players try to use this rune, and it really just isn’t for Kassadin considering how many times you will riftwalk without wanting to actually get that movement speed.

Here are my favorite options for Inspiration in the sustain 8.21 meta.
Sustain meta
Sustain meta
Updated in Patch 8.22: With changes to Time Warp Tonic, it may make this rune page weaker; however, you can still take these two runes for extra sustain early. I have not found to be reliant on these runes for sustain, so other options in Sorcery and other pages may be more useful this patch.

Here are my favorite options for Domination. It's a pretty ballsy move, and I like you for considering it.
Sudden Impact is a solid choice for extra burst and trades, and if you are taking any Domination Page with Kassadin, this is a great choice.
A good choice for vision control and extra AP. Completely situational and depends on your playstyle.
A good basic rune for just doing more damage. If you take this rune, make sure to roam post-6 to get stacks on this.
Not bad for roaming and more mobility. I like this rune in general, and on Kassadin it allows him to roam more.
Don't. Same story as Nimbus Cloak

These are my basic rune choices. To sum it up, Electrocute and Fleet Footwork are the most practical and useful primary rune pages, and secondary rune pages depend on your play style, although Sorcery lets Kassadin's highlights really shine. Again, my basic rune page is at the top of this build.

There are many options to take for Kassadin, but here I will talk about ones that will help him scale with this build

First off, I like to start Sapphire Crystal and Refillable. There are other options like Corrupting Pot or D Shield or D Ring. These options aren't bad, and Corrupting Potion is actually a pretty good start in this meta, especially with the Inspiration secondary that I mentioned; however, I like Sapphire Crystal + Refillable because it: 1. Allows you to get a tear 350 gold earlier in order to stack as soon as possible & 2. Allows you to take poke very early on, heal it up, and when you back you still have potions when you go back to lane with Tear.

Next, we have our core items. Once you finish your Tear, instantly go for Rod to start stacking quickly. Often times, I like to get the Catalyst first instead of the Blasting Wand because with low cooldowns, having more mana and hp just makes you better. Next, you want to finish your Archangel's Staff as soon as possible because of the extra 20% CDR and the extra AP and mana you get, but often times you will need a little bit more time to finish stacking your Tear. Somewhere in between here, depending on how much excess gold you have, make sure to purchase Tier 1 Boots.

After finishing your core items, you can start deciding what boots you want. Here are my favorites:

I like Sorcs because the extra magic pen is good in general on an AP champion. I would take this if I'm ahead or if they don't have many threats that you can't take down.
Mercs are good vs a team with CC that can shut you down, and especially if their AP Carry is ahead.
I would not recommend this item in most situations, but it can work at times. If an enemy ADC is ahead, going sorc shoes is good to shut them down easily, but if say an Irelia is ahead and they have an AD jungler or mid, then tabis aren't a bad choice.
CDR Boots aren't the worst thing you can build, but you will be getting 40% CDR either way, so they are much worse compared to Sorcs or the other options.

After boots and your core items, there are many items to choose from. You can really get any AP item, but here are my favorites ranked from first to last.

Lich Bane is by far my favorite item after the core items on Kassadin. It just offers so much damage, not to mention the extra CDR and Mana. All around my favorite item to build when average/ahead, but if the enemy is doing lots of damage, there are better options.
Zhonya's is a great option vs a heavy AD team. It gives armor, AP, CDR, and a nifty passive that you can use. If there are other options like Lich Bane that you want, then at least getting a Seeker's Armguard is a great option.
Morello's is a great item for damage. The magic pen it offers is amazing and it also gives a generous amount of AP. It also gives HP which benefits Kass more than other AP Carries, and the Grevious Wounds is an extra touch that helps. If you are going for pure damage, this is a solid option
Void Staff and Rabadon's Deathcap are great finishers to an AP build, so this is a great option; however, if the enemy isn't building magic resist (which they would if they were smart), then flat magic pen items like Morello's or Liandry's are better. Still a solid option vs tanks and magic resist
Not too much to say about this. Offers an insane amount of AP and damage.

Starting Spell Sequence
1. Q
2. W or E
3. W or E
4. 2 points in Q
5. Max E
6. Max Q
7. Max W

Taking W or E second or third is very situational, so here are my two main tips:
If the enemy is a hard poke champion early or you can't really touch the wave then E second is better
If the enemy is melee without much poke or the wave is pushing to you/your tower then W second is better

I Like to put a couple of points into Q early for the nice CD and damage, but getting E maxed at level 10 makes level 11 a better power spike for you, considering how much damage maxed E does.

Combos
Level 1-6 you can poke with Q and E, but there isn't too much room for combos considering you will be playing safe
Post-6 you can do a quick combo for maximum damage depending on the situation
1. If the enemy is in a safe range and you can catch up easily then Q > R > W > E
2. If the enemy is in E range, but they are a little farther and you need to catch up then E > R > W > Q
3. Once you get Lich Bane, you can go in for a quick R > W for a lot of damage then follow up with E & Q
4. To clear the back caster wave, you can usually 3rd or 4th stack Rift Walk to completely kill it, or you can R > E and they will die.
Remember to use Rift Walk creatively to increase your damage!

With a Sapphire Crystal and Refillable start, make sure to play levels 1-6 very safe. Try to let the first two waves push into your wave, and if the wave is starting to push, make sure to level W second. Kassadin is quite good at farming under tower with his low cool downs and effective auto attack cancelling with W. Also, it is hard to get poked down while farming under turret. Try to use Fleet Footwork procs to restore hp as much as possible. Back exactly at 500 gold to get Tear and use Teleport to get back to lane. You should miss 1 maybe 2, 3 maximum creeps. Make sure to use spells whenever your Tear is off cool down to stack quickly.

Once you get Rod of Ages, you should try to finish boots. With Sorcs, you should be able to roam bot lane. Team up with an Alistar or Thresh for an easy gank.
Fighting mid lane shouldn't be too hard, as long as you don't get outplayed. Versus AD match ups, fighting isn't ideal, but versus an AP champion, especially mages, you can easily 1v1.
Late game, you should be taking as many even fights as you can. Try to roam into the enemy jungle and kill the jungler while they are low. Escaping the jungle should be easy using Rift Walk Rift Walk well.