AP assassins in League of Legends have a weird way of feeling as broken as possible, at least in the old pre-Black Cleaver days. Katarina right now is the hot issue, Twisted Fate is still a little frustrating to deal with, LeBlanc either curb stomps or is useless, Diana was straight-up broken since release and only radical chainsaw surgery saw her drop out of use, and back in my day Kassadin was the one everyone feared. The fact that he has a free flash was terrifying, and he soaked magic damage like he was made of rubber.
|Eats magic damage, gains aspd||Fragile early game|
|Built in flash||Not great vs AD|
|Q hits like a truck||Squishy|
|Free mana returns||Q is expensive|
I don't remember a single thing about early, early Kass. I didn't play before he got some tweaks to his kit and it became the magic-consuming AP assassin everyone feared for what feels like forever. What I do know is he's basically done what he's always done – hung out in mid lane, swallowed the Kat or Brand who picked ahead of you, and turned the side lanes into a nightmare.
Right now, he's all but vanished. Resource-light champs like Kat or AD assassins like Kha and Zed have come to the forefront with all the changes to armor/magic penetration, Black Cleaver's remake, and the snowbally nature of Preseason 3. It's just how the game has turned, and though he's still got that scary midgame roaming power, someone like Kha'Zix or Talon can demoralize the opponents before they even know what happened. He has, ironically, taken the position Talon used to be in.
So, he's not a strong pick at the moment, but he's fun as all hell, especially as a counter. Run him against any hard-hitting burst mage or assassin, such as Elise, Annie, Anivia, or the like. He easily curb stomps Katarina if you know how to run him, his Q outranging her harass and giving him a hard silence to shut her ult down with. And Karthus is downright hilarious – it's not easy to passively farm near your turret when Kass can eat any damage you pump out and nuke the ultrasquish mage from afar.
Problem is, if the lane is unconventional, it tends to ruin him. This is most strong with AD mids, denying him his magic-eating passive, which is why he's kind of a terrible choice right now - I haven't run him against Zed yet, but I'm betting with the energy resource and the skillshot Q, it won't end well for Kass. Talon completely ruins him, with a longer range harass and higher damage output, Gangplank has enough durability and a better poke to just whittle him down, and the ever-obnoxious Galio Mid pick does what he always does.
Being an AP assassin with single-target nuking power, his build is what you expect it to be – lots of AP and spell pen. Sorceror's Shoes, Deathcap, and Void Staff. The items you want all depend on what kind of game you're running. Let's say you get an easy counterpick and have puttied your mid lane, you want to go for a Lichbane to get that extra slap in, and a Deathfire Grasp to make your burst hurt even worse. If your AP mid is giving you issues, Neg Cloak into Abyssal gives you more magic resist and damage, and a Tear of the Goddess into an Archangel's gives you easier farming. His Q is expensive, and his ult practically never stops increasing in cost. If you get zoned, you have to rely on the Q to farm with.
His laning feels kind of standard, or maybe that's just the Gangplank addict in me talking – jab a lot with Sphere, and if you can bully them out of lane, push the wave and drift top and bottom for bonus kills. His Wave harvests nearby spells to prepare itself, and it's effectively a useless skill when not ambushing.
Problem with Kass is that his earlygame is pretty difficult to handle. He's got a super expensive Q, and he's melee, so you have to find a delicate balance between farming, harassing, and not getting harassed. He runs his mana pool dry if he gets zoned by someone like Gragas. The current top strategy is to pick up a flask and potions and just farm and soak the harass. Once you get out of level 1-4, everything is just uphill. And that's a co-writer Brandon Law strategy, who keeps reminding me how much better he is at this game than me (I say that affectionately, of course).
Regardless of what you get early, more mana sustain for him always helps out immensely with his tiny mana pool in case you need to start farming with his Q. Catalyst and Tear are great earlygame, the former if you find yourself getting harassed a lot or the latter if you want to build up a huge buffer of mana for more Sphere spam. Remember too that the new Archangel's turns into Seraph's, which gives you a surprise shield if you need to get out in a hurry. And Catalyst builds into Rod of Ages, which grants a little of everything – more tankiness, more mana, the sustain, and more AP. Your priority is to do as much damage as possible, but even Talon does well with a bit of durability.
Remember that roaming is probably Kass's chief advantage. He lacks in a hard CC, but he's got an area slow and a free flash, which (no matter how crazy mobility creep gets in League of Legends) is ridiculous. This is why Kass was feared for so long - if you get ahead in mid, drift over the wall to side lanes via red/blue buff, wait for a good advantage, flash over the wall, and melt whoever dies fastest.
Assassins do their best work during midgame, which is why the little meter I sketched up at the top indicates that his best time to work is early-midgame. I could give you this advice for anyone, but if you play an assassin mid, you need to get ahead and use your power to win the side lanes. This is why Kha'Zix is dominating at the moment – he's got a nasty slow, a gap closer with refresh powers, and a spammable Q, not to mention armor penetration is the hot topic of the month. Kassadin, if he can get past his rough early stage, does well with this.
When you get to late, Kass tends to slip downward. He can still liquefy targets, but he doesn't have the CC of someone like Annie or Lux, nor – without an ult that does damage – does he have the same destructive power of someone like Katarina or Brand. He shines when you can appear and quickly silence a soft target. In big teamfights, he loses some of that presence.
But, like most assassins, if you hold out for a teamfight, and wait just a moment longer, you can slip in and clean up the fight. If you don't, chances are you will get focused first, and if you're still powerful, you want to make sure you do your damage early. Consider a quick Guardian Angel, or if you have a Yorick, hope you get lucky.
Kass might slip a bit until they handle some of the insanity going on in mid lane, but he's still as strong as ever. If anything, his kit is still nasty. Talon and Panth and Zed might benefit the most from the new items, but neither have a flash on a five second cooldown. He has a high skill cap, but if you can get him out of early laning alive, you can eviscerate.