Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Okay, first of all, let me say this, and i can't stress it enough: this is a BUILD not a GUIDE, also everyone is entitled to their constructive opinion and voting, however i would like you all to first think what this build builds on and vote afterwards.
I had quite a few kassadin encounters in my day, and i think i never saw one quite like this, everyone either went pure AP or didnt really shine.
My observation, however was, that kassadin, even if you do not add a lot into it, can hit really hard, since his W skill is quite cooperative to that, and everything else is either a ninja teleport, disable or massive slow. I think, and i have played this build before, that every build needs adaptability, as to how you progress in early to mid game.
Open into saphire and some potions. you do not need the potions if your co-laner has heal, is a healer or generally knows how to team play. At this point you could also open into vampiric scepter and be a jungler, allthoug possible, kassadin is really good in a lane, so lets keep him in one for the time being. As soon as you farm enough gold, use a force pulse on the creeps, and back to buy tear and boots. If it happens you have too much money, but nothing good to tech (let's say you have 700g, not enough for a blasting wand but still a lot), upgrade the boots, stock up on the consumables (preferably only up to level 8-ish, after that maybe a blue pill or smth, i dont care, really) and head back. At this point, if you find yourself unnaturally owning everything in your path, instead of getting the usual stand back and try not to get harassed treatment, go for soulstealer insted of the tear.
If it happens that there is a good gank available, take it. Don't be shy in later early game, that is after you have tear and t1 boots. If you manage to squeeze the tear/stealer in before early game is done, good for you, however unlikely.
Now, that you are properly fed, get yourself the archangle staff. If you find yourself facing a fatty team (basically more tanks, less damage), you can go into madreds bloodrazor tech instead archangle staff tech, and also jungle really well, by mid game. If its a normal team you are facing, go for archangle's staff or into rageblade, both need the blasting wand, so that is the first item you go for in a normal mid game start. By now you should figure out, that only spamming spells doesn't cut it, so, you decide to cut some stuff yourself. Don't be afraid to get close and whack someone, you can also take the W skill upgrade faster, if you think you need to, it helps versus fatty teams.
Your gorals for late mid is to either have Lich bane nearly done or nashor's tooth finished or near completion. If it so happened you now have madred's insted of archangle staff, take nashor's tooth first.
By now, most people already GG, since it's late game and people tend to be noobs like that. If the game is still going, procede to get Lich Bane finish/tooth or, if you went Madred's, just go for MAlady, since it works great with your hero, or go phantom dancer, if you have the dough.
Some people rant that this build is squishy, so i recommend a hextech gunblade or bloodthirster (i added the gunblade since some people play it with madred's i think they think it stacks, either way, its still nice, if you manage to use force pulse correctly.). I recommend you leave out soulstealer in this case, since if you need healing, you probabily arent exactly roflstomping the enemy heroes. I also think that a viable combo for kas' is using manamune instead of archangle's staff, but that makes the build really AD focused.
Im not saying this is the best kassadin, anyone ever played, but i can tell you, you can kick some serious *** with a hybrid. Give it a test, and leave a comment, i might tweak this in the future, depending on the response. I might even make an all out kassadin guide, for what to get, or what's an absolute no-no.
P.S.: I will make a revision of the build, and make it fancy when i have the time, currently swamped beyond measure, so i have 0 time.