In the early game, this will be more important than Force Pulse. Use this spell to harass and last-hit minions. Late game this will be your nuke, dealing around 500+ damage (before magic resist calculations). In team fights, your job is to silence and take out their caster in the back. Also, use this spell to stop ults (Nunu, Katarina, Shen, etc). Be smart and save this for the right time.
Force Pulse will be your main damage dealer, dealing more than 600+ damage. This ability has many, many uses. First of all, the main use for this ability is for the damage (of course). Eating away more than 800 hp from an enemy is just orgasmic. Second, the slow effect is very useful when chasing an enemy. You slow him, teammates catch up, you guys buttfuck him. Keep in mind that Force Pulse will still hit them slightly outside the cone of range. Lastly, it can be used when trying to escape. When you're running, Force Pulse them and then Riftwalk. They'll be slowed for three seconds and you'll be ahead 20 feet. TRY TO HIT AS MANY TARGETS AS POSSIBLE. There is no damage reduction depending on how far the enemy is (like Pantheon's ultimate).
This is the reason why you play Kassadin. It's fun, effective, and easy. This should be your opener MOST of the time. Sometimes you're gonna want to save it just in case the enemy flashes or blinks away as soon as he sees you. Also, keep in mind that casting consecutive Riftwalks will completely demolish your mana. Once you cast Riftwalk, you'll get a timer that lasts 8 seconds and will reset back to 8 if you cast another Riftwalk. The more stacks you have, the more damage it will do at the cost of 100 extra mana. For example, let's say you're chasing Master Right-Click. He's at 30% hp and he pops Highlander and starts running. You have 700 mana left. You cast Riftwalk to catch up, leaving you with 600 mana. You still need to get closer, so you cast another Riftwalk as soon as it comes off cooldown, leaving you with 400 mana. Still not close enough, but you know that one more Riftwalk will get you right on his ass, you cast another as soon as it's up. You finally catch up to him with 100 mana; now what do you do? Keep and eye on your stacks and try to keep your Riftwalks below 3 stacks (that's 600 mana). If your chasing and you know you're gonna get the kill, then go for it; if you think you won't have enough mana for a combo, wait for the timer to expire, then cast again. For traveling, never stack your Riftwalks. Use it, wait for the timer to expire, then cast again.
Do not Riftwalk into the entire enemy team thinking you're Rambo. Wait for your tank to initiate, then pop up behind the casters and make them eat your lavenderpoopoo.
Also, if you think an enemy is in the bushes, don't Riftwalk into it. I see a lot of Kassadins make this mistake. You're using your escape mechanism as a bush-checking opener. Save you Riftwalk just in case you end up running into a trap.
Note: Try not to overuse this ability in the early phase. Later on in the game, you won't have to worry about mana problems thanks to your item build.
Item Start x2 -OR- x2
Core Build - totaling 8325 gold
Since Kassadin's Riftwalk can stack and his Force Pulse needs to charge, I like to think of him as a snowball champ. He is very item-dependent and will need lots of AP, which is why you're gonna buy a Mejai and Rod of Ages. This basically means if you suck ass early game, you will suck major ass later on. Be very cautious early game.
Kassadin was made to counter mages, just like Jax was made to counter DPS. His Riftwalk allows you to move over the rough teamfight and reach the squishy ones in the back. The silence ball will render them useless for up to 3 seconds, giving you plenty of time to poop on their face. The basic combo is: Riftwalk in > Force Pulse > Null sphere finish them off (not too hard, just a couple of auto attacks and ignite) > then Riftwalk out.
A good team will pick up your tactics real quick and will aim their fire at you as soon as you pop out, especially if you have stacks on your Mejai's Soulstealer. So when the team fight breaks out, wait for some sparks to fly, then shit on the casters. Also, you won't have to worry about not having enough stacks for your Force Pulse. In teamfights, 20 abilities will fly over your head, so your only limit is the 5 second cooldown.
As an anti-caster, you're going to be dealing with a lot of spells in your face. This is the reason for packing magic resist. This will leave you vulnerable to heavy DPS (pretty much your only weakness). But, that doesn't mean you're useless against them. You still have TONS of burst with very short cooldowns and an escape mechanism. Also, if you follow my item build, you will have over 2400 hp, making you a lil more tankier.
One of Kassadin's biggest problem is his mana in the early phase. I can't even count how many times clarity had allowed me to use one more combo to finish someone off. And with such a low cooldown, you'll be able to stay in a lane as long as you want with a couple of health potions. As for ignite, I use it for that extra 'oomph'. Never hurts.
I've heard that Clarity is pretty bad on Kassadin. I disagree because he is VERY mana-hungry early game. Also remember that you don't have to take my summoner abilities, I'm just showing you which ones I use and seems to work.
Other good ones:
Exhaust is a great spell because it works well with your passive (35 + 10 = 45% reduction from magic damage.) Also, you add another slow ability to your arsenal. You'll be able to slow an enemy for 6 seconds, which should be just enough for you and your partner to kill in the early game.
Heal I've seen some Kassadins use this. Early game I can see this as useful, but later on, you will have over 2000 health. You shouldn't even be taking any damage since you're supposed to hide in a bush, Riftwalk in, kill, Riftwalk out. As long as you're careful while you're laning, you won't need this.
Ghost is really nice on Kassadin because you already have a blink. This will save you from ganks before level 6. If you're completely new to Kassadin or you're the kind of guy who tends to die a lot in the early phase, take this ability instead of ignite.
Cleanse A really good spell for any champion. If you get CC'ed, pop this spell, then Riftwalk out.
Teleport Good for helping with ganks and defending turrets. I hardly get this, though.
[spell=fortify] - Kinda useless. Invincible turrets for 6 seconds, with a 5 minute cooldown? No thanks...
[spell=rally] - Also kinda useless after the nerf. Have someone else take this.
Smite - Good for stealing buffs and doing Baron late-game, but I dont recommend it. The only reason you should be in the jungle is if you're switching lanes to gank, chasing someone in the jungle, or grabbing golem.
Masteries won't make a huge difference since they're only a small boost, but i recommend: 9-0-21
You will need magic penetration and mana regeneration as well as reduced summoner spell cooldowns. A Clarity on a low cooldown will keep you in a lane forever. You will also need more movement speed since Kassadin is very vulnerable to ganks before level 6.
As for runes: Greater Mark of Insight - Never EVER get AP marks over MPen. Magic penetration runes will give you more burst than ability power.
Greater Glyph of Shielding - You will need magic resist. The reason for taking per level runes is because most casters won't be much of a threat early game.
Greater Quintessence of Swiftness - 340 movement speed without boots, 392 movement speed with Boots of Speed, and 413 with Mercury's Treads (with masteries). Your movement speed is one of your defenses in the early phase.
A rank 1 Null Sphere will do more damage than a rank 1 Force Pulse. Same thing with rank 2. At rank 3, both abilities deal the same amount of damage. Force Pulse becomes better than Null Sphere when you've skilled it to level 4 and have some ability power.
This skill order maximizes Kassadin's Riftwalk-Force Pulse-Null Sphere Combo at level 6.
After level 6, it's best to shift your spell priority to Force Pulse because it has a higher ability power ratio than Null Sphere.
Well-Rounded: Lich Bane More mana for your Staff, more magic resist, more ability power, more movement speed, and another nuke to your arsenal, dealing physical damage equal to your ability power (550+)
Start off with a Sapphire Crystal or a Meki Pendant and 2 Health Potions. Work your way towards a Tear of the Goddess and a Boots of Speed (you won't be finishing these boots til later since you'll be at 392 movement speed with runes and masteries). You want to start feeding your Tear as soon as possible, since this is one of the main sources of your huge mana pool. Once you have enough gold, grab a Rod of Ages starting with whatever item tickles your fancy ( I usually grab the Blasting Wand and the Ruby Crystal for more damage and some survivability). Once that's complete and starts charging, go treat yourself to a Mejai's Soulstealer. The reason you should get the Rod before the Soulstealer is because it takes 15 mins for Rod of Ages to fully charge, so you gotta get it as soon as possible. If you get this item around the 16-22 min mark, you're right on track. Kassadin shines mid-late game, which is when you should have your Soulstealer by then. Oh and don't worry, you'll get kills. Once you have your Tear, your Rod, and your Soulstealer charging, turn your boots into a Mercury's Treads since the enemy casters have lots of damage by now and because you will be vulnerable to CC. Once you've strapped on your new shoes, finish your Tear into a Archangel's Staff. By this time, you should have over 2k hp, 2.5k mana, decent amount of magic resist, and a shitload of ability power. The game should be close to finishing by now; if not, buy the last items depending on the enemy team.
Whenever you go shopping and you have an extra 250g buy an Elixir of Brilliance, but not before your Tear of the Goddess or Boots of Speed. Also, health and mana potions are not just for the laning phase. Be sure to buy 2-4 health pots and 1-3 mana pots whenever you can. Don't spend too much gold on pots, however. Keep in mind that Kassadin is dependent on his items. Don't let pots and elixirs delay your build. Just buy them when you have extra gold after buying an item.
Against Heavy DPS Team - 11400 gold
This is where Kassadin is most vulnerable. You will be almost defenseless and mana-hungry until level 6. On the bright side, your Null Sphere will be hitting pretty damn hard, so harassment will be a breeze. If you want kills (troubles with farming minions), you will need a good lane partner. Preferably a champ that can charge your Force Pulse and dish out some damage (i.e. Karthus, Ryze, Pantheon, etc.). I prefer Pantheon because he has a very strong combo as well, including a stun.
I HIGHLY RECOMMEND THE MIDDLE LANE. Kassadin works just fine as the solo laner. For some reason, I do better in the middle lane than I do with a partner. Just harass and farm while also being careful. By the time you reach level 6, they should be standing at around 30-50% HP from your Null Spheres. The second you reach level 6, learn Riftwalk and execute your combo. You gotta be quick because as soon as they see you turn 6, they will be extremely cautious and start to turret hump. Riftwalk > Force Pulse > Null Sphere > Ignite(if you have it) will result in a kill 95% of the time.
If you didn't take the middle lane, be cautious and last-hit minions. Don't even auto-attack; just last-hit. The idea is to let the enemy push so that the minions will be fighting just outside your turret's range. You don't want the minions too close because you'll be fighting your own turret for last hits. With this strategy, you'll be able to farm in peace without worrying about ganks. Also, if the enemy champs try to harass you, the turret will target them because the minions are out of range. Do not neglect harassing them, though. Tell your partner to focus on the squishier one. Once they're at around 40% hp, throw a Null Sphere and spray your Force Pulse while your partner attacks them. Finish them off with ignite if it's necessary. This will net you a kill most of the time. (Don't try to harass enemies that have lots of HP, magic resistance, or heal spells. In that case, save your Null Spheres for minions.)
Be extremely careful and keep an eye on your minimap. Try not to go past the river before level 6 if an enemy champion is MIA. Once you reach level 6, your only defense is your Riftwalk, your Boots of Speed (392 movement speed with runes and masteries; 375 MMS with just masteries), and your reflexes.
Don't be too aggressive. Kassadin deals burst damage, not sustained. Once you've blown your combo there's not much you can do right after except auto-attack and wait for you cooldowns to finish.
Basically, all you want to do, is control the minions and farm.
Working in the team
As stated before, your main job is to take their casters down. Wait for the fight to start, then Riftwalk to the squishiest caster and unleash hell. Your Riftwalk will have a cooldown of 2.4-6 seconds (depending on your level and if you have 20 stacks on your Soulstealer and an Elixir), which is just enough to finish them off. Riftwalk out. Wait for your Null Sphere and Force Pulse to come back up (5-6 seconds), then repeat. Pretty simple.
Positioning is very important before the teamfights. Try to hide behind a wall or in a bush behind them. Once the fight starts, Riftwalk in behind the casters and take em out. Try setting your mind to "ninja mode". Basically, hide in the shadows and wait for things to start. When you see an opening weakness, exploit it, then disappear. =P
Pay attention to your enemy's summoner spells. If they have heal, immediately ignite them after your Force Pulse. If they have flash, save your Riftwalk. If they have exhaust, Riftwalk away, let exhaust expire, Riftwalk back in.
Also, you are well-suited to pick off any runners.
Creep Jungling and Farming
Not much to say here. Early game, just try to last-hit the minions with auto-attacks and Null Sphere and get as much gold as possible. As for creep jungling, you WANT the golem buff. This will cover your mana problems. You want the golem buff AT ALL TIMES. Steal it from an enemy or even a teammate if you have to.
Getting golem isn't easy since Kassadin is an awful jungler. Ask a teammate for help.
Signs of a bad Kassadin
Here are things that will make you look bad:
-Focusing your burst on someone other than the enemy mage
-Riftwalk-turret diving into an enemy champ that has a CC (Annie, Morgana, Sion, Shen, Rammus, Taric, etc.)
-Spending too much gold on excessive elixirs and potions
-traveling with stacked Riftwalks
-not silencing heavy AOE ults
-initiating fights with Riftwalk
Increasing the range on Riftwalk
Basically, just Riftwalk onto a large wall. You obviously cannot blink INTO the wall, so the game mechanics will just place you on the outside of it, making you Riftwalk farther.
Traveling with Riftwalk
Just cast Riftwalk, wait for the timer to expire, then cast again. Nothing much here...
Pros / Cons
-Hundreds of ability power
-Extremely huge mana pool
-Incredibly tough magic resistance
-Deadly burst damage
-Very short cooldowns
-high ability power scaling
-has one of the longest silence ability in the game
-has one of the most effective AOE slow in the game
-has the best blink ability in the game
-viable in any lane
-small defenses and squishy during early game
-a lil mana-inefficient early game
-mediocre damage early game
-somewhat vulnerable to DPS
-dying can be costly