Hi, I'm Cheesecake404, some random low-elo scrub who likes poros, making his first guide here.
I have a decent amount of experience with Kassadin to make a beginner's guide. You aren't here to see who or what I am, so let's get to the guide.

Before we get to the specifics of Kassadin, let's first talk about what he's good at. After all, I'm pretty sure you want to know if he fits your playstyle or not.

Positives

•STRONG Late game.
•Great splitpusher.
•Can One-shot Squishes.
•Can peel decently (Based on Build).
•Is a very good 1v9 Champion.
•Very versatile for different playstyles.
•Good into almost every mage in the game.


Negatives

•Extremely bad into AD assassins.
•Very Weak Earlygame.
•Can be hard CC'ed into oblivion.
•Often the first targeted due to diving into teams with ult.
•Terrible Wave clear early.
•Very vulnerable to ganks in laning phase (1-10)
•Can be shut out of the game without farm.


Overall, I believe Kassadin to be very good in the current meta, and pretty strong to one trick or just to have in your pocket to counter certain mages you have trouble against with your other champs.

Kassadin has some interesting abilities, such as him being level 16 makes him the most mobile champion in the game because of his ult. And probably one of the strongest as well.

These descriptions are from the Fandom of Kassadin.
Numbers from the official League Of Legends Website.

Passive: Void Stone

Kassadin takes 15% reduced magic damage and ignores unit collision.

His Passive is pretty useful against mages and to stop being minion blocked, unit collision is for nerds.

Q Ability: Null Sphere

Kassadin fires an orb of void energy at a target, dealing damage and interrupting channels. The excess energy forms around himself, granting a temporary shield that absorbs magic damage.

Damage: 65 / 95 / 125 / 155 / 185 (70% AP Ratio)
Magic Shield: 60 / 85 / 110 / 135 / 160 (30% AP Ratio)

This ability is pretty useful for trading in the early game and for farming into a Talon/Zed or any other hard matchup for you.

W Ability: Nether Blade

Passive: Kassadin's basic attacks deal bonus magic damage. Active: Kassadin's next basic attack deals significant bonus magic damage and restores Mana.

Basic Attack Bonus: 20 (10% AP Ratio)
Bonus Magic Damage: 70 / 95 / 120 / 145 / 170 (80% AP Ratio)
Mana Restored: 4 / 4.5 / 5 / 5.5 / 6% of missing mana

This along with your Q can help you trade without worrying about mana. Fun Fact, this also resets your aa timer, so you can aa, w, aa. This with Lich Bane can nuke people for a TON of damage late game. Keep that in mind.

E Ability: Force Pulse

Kassadin draws energy from spells cast in his vicinity. Upon charging up, Kassadin can use Force Pulse to damage and slow enemies in a cone in front of him.

Magic Damage: 80 / 105 / 130 / 155 / 180 (80% AP Ratio)
Slow %: 50 / 60 / 70 / 80 / 90%
Abilities needed to be used: 6

This ability is decent early game for some combos like E,Q for trading or an level 6 all in. Otherwise, its a solid slow and late game the ability cost is a non-issue as most of the time it will be up.

Ultimate: RiftWalk

Kassadin teleports to a nearby location dealing damage to nearby enemy units. Multiple Riftwalks in a short period of time cost additional Mana but also deal additional damage.

Magic Damage: 80 / 100 / 120 (40% AP Ratio) (+ 2% maximum mana)
Magic Damage per Stack: 40 / 50 / 60 (10% AP Ratio) (+ 1% maximum mana)
Mana Cost: 50 / 100 / 200 / 400 / 800 (Based On Stacks) (At All Ranks)

I didn't go over the mana cost for the other abilites as they were mostly normal, but this one relies on it so I'll talk about it. You REALLY need to keep watch to make sure you don't run out of mana or hit max stacks in the early game, or you will be a sitting duck ready to die to the 10/0 Talon. Otherwise, This ability is what makes Kassadin himself. It gives him the best mobility in the game after 16 and does stupid amounts of damage, AOE damage no less.

Fleet Footwork

This rune helps you sustain in lane with its health and ms in any matchup, which makes it Kassadin's best keystone rune in my opinion and many others, though some other Kassadin mains may prefer Electrocute.

Presence of Mind

A great rune practically made for him, it helps him instantly gain his mana used from his ultimate in team fights and skirmishes. Practically essential if you want to spam his ult damage in a teamfight as you will quickly run out of mana.

Legend Tenacity

Most of the Legend runes, don't help Kassadin very much, but this provides him with cc protection for when he is jumping in, helping him in teamfights or chain-cc.

Coup de Grace

Just helps Kassadin burst squishies faster and does quite a bit of damage.

Taste of Blood

A rune that can help you survive into most matchups, along with Fleet, make you very hard to push out of lane with the healing.

Ravenous Hunter

Another healing rune that scales well into the late game, giving you tons of health when fighting. Just simply amazing at keeping yourself alive once you get max stacks.

For Minor Runes you take Adaptive Force, Adaptive Force, and MR/Armor based on matchup

Kassadin has many things that need to be realized before playing.

He is a able to flash during his ult's startup. You can use it to catch out enemies that are far away, and one-shot ADC's in team fights.

After your ult, you should if in range, stab them with your W, and then go into an E, Q. Since your Q's range is bigger than your Force Pulse.
R, W, E, Q

Other than that, I believe that this is the end of the guide. Thanks for reading and please suggest stuff if you want.

I'm going to be updating this guide with more matchups and probably adding a combo section later, so I'll be back.