These summoners are almost always the best options.
is a spell that gives you kill potential as well as lane pressure. Even if you are in a lane where your kill potential is very low, one of your jobs as a mid laner is to roam and should help you net kills when you gank.
For instance, if you are level 5 with level 3 Q and your opponent is at around 400 health, they are forced to port back. If you had or any other summoner, they could stay in lane and eat more poke without fearing death. In solo queue, heavily outshines .
However in games where everyone is on the same page (Team 5x5), could still prove to be very useful and is definitely a viable option.
Imo, and can be okay alternatives if you are really afraid of being burst down.
is not my cup of tea. If you need to escape from deadly cc, build a .
Greater Mark of Magic Penetration This mark is the best option on many AP Champions. They not only give you early game power, but scale very nicely into late game. On Kassadin, this is honestly the only option.
Greater Mark of Hybrid Penetration
People have started to use hybrid penetration marks more frequently. They can be useful on a few champions, but Kassadin is not one of them. You should NEVER run these on him. Before you decide that these runes are
viable on Kassadin, take these factors into consideration:
Kassadin rarely auto attacks during laning phase, the stage of the game when these runes shine most on ranged AP mids like , , etc.
Kassadin's nether blade (W) which makes his auto attacks part of his damage source post-ultimate during fights is all magic damage.
Runes are most important in the early game, when every bit of magic penetration helps your Q hit harder.
Late game, your ad damage is very minuscule, and the extra damage from armor penetration will probably be in single digit numbers.
Greater Seal of Armor This seal is the best option for Kassadin for all matchups. Here is my case:
As a melee champion you're forced to farm under auto attack harass from ranged champions. Don't underestimate this source of damage as auto attack harass could add up to a LOT of damage. Farming against AP mids with ranged auto attacks can be a nightmare without these seals to help you reduce damage in this department. Running these against AD mids is a no-brainer.
Armor helps you reduce minion damage. This damage can add up to quite a bit if you are put in a situation where you need to take some minion hits.
Lane swaps. Kassadin's worst nightmare is not pantheon nor talon, but a lane swap bringing an ad carry and support mid. While usually a rare occurrence, 1v2ing which is already brutal will become impossible without armor.
Greater Seal of Scaling Health
If you feel that you absolutely won't need the armor in lane, this is probably the next best option on Kassadin. Mana regen is unnecessary as you will be starting flask and rushing tear as your first item, and these will give you about 175 health at level 18. While this might not seem as much, That's about as much health as you get from a ruby crystal which isn't too bad at all. The only part that bothers me about running these runes is that it is really hard to find a situation where you won't be benefiting from running armor in one of those 4 situations I listed above in the armor yellows section, and by level 18, the role of runes really dwindles down.
Greater Glyph of Ability Power Using AP Glyphs strengthens your early game. These glyphs will help your spells do more damage and even against tanky matchups, the damage from Q should build up to a moderate amount. Even against AP champions with high magic damage, you should run these runes to shove them out of lane over time and keep your health high through flask and potions. Once you're level 6, you become a much stronger champion and you should start to win the lane even against hard matchups if you survive through that phase.
Don't forgethelps you survive against AP damage as well.
Greater Glyph of Magic Resist If mid & top or mid & jungle both are primarily AP damage dealers, that calls for Flat MR blues. Don't ever run scaling magic resist. If you want to run scaling magic resist, run the flat AP instead.
Greater Quintessence of Ability Power These quints add up to around 15.6 flat AP with your passive. That amount of AP is nothing to scoff at, as it's pretty much 3/4 of an . This should improve laning, increasing kill potential and lane pressure throughout early game. I believe running magic pen on quints hurts your early game damage too much for it to be worth it, as runes are mainly for the early game benefits.
Greater Quintessence of Movement Speed
These quints are viable to help you against matchups that require you to avoid skillshots. Some champions that come to mind are , , and who have skills that can punish you extremely hard if you are hit. Even though Kassadin has a blink, it won't come until 6 and these may be what you need to survive that early laning phase.
Never change the 21 points into offense. The only possible change you could make is to take out 2% from cooldown reduction and put it into butcher to help you last hit. You need the 21 in offense to deal as much damage as you could out of your masteries setup. No other setups are worth losing the 5% increase in AP (Think of it this way: If you have 300 AP, that's 15 AP from 4 points. If you have 600 AP, you gain 30 ap from the 4 points.) and the 5% damage increase to anyone with less than 50% health (If you deal 1000 damage to a target with less than 50% health, that's 50 free damage from 1 mastery point [Even with resistances, I believe any extra damage in double-digit numbers is a huge gain for 1 mastery point]). You also gain 6 AP right off the bat which is a great addition to your AP from runes. Spellsword is also a nice 1 point mastery which helps you deal extra damage with W.
If you decide to go with 9 points in defense, you should take the 4 points in durability[6 Health per Level], whatever resist you need + 1 of the other, and veteran's scars [30 Flat HP].
Utility is where my 9 spare points usually go to. It not only gives you 15 seconds off flash as well as a 10% reduction both your summoners, but it gives you mana regen and extra buff duration which are both valuable assets. While the 15% reduction in active items may seem like a good idea, the active items you build are both very situational actives and it's very unlikely you will use them as soon as they're up again. While same could be said for your summoners, the cooldown is much longer and I believe that reducing the much higher cooldowns of summoners is a better use of the mastery points.
Always pay attention to your matchup and change your masteries accordingly. Also, have a playstyle based on where you spend your 9 spare points. This is very important. If you take mana regen, don't be afraid of spending mana if it will win you the lane. If you took 9 in defense, understand you took defensive masteries because you intended to play safe or you wanted to trade more efficiently and play aggressive through favorable trades.