Not Updated For Current Season
I have been tinkering around with hybrid builds for a while lately out of boredom of going oom whenever i want to kill someone. I started using this one a few days ago and only went negative in 1 game with it so far (it would be 0, but a newbie vlad jacked mid from me and ruined everything >.< and this doesn't mean that poo-poo teams didn't cost me victories in spite of my efforts).
Basically your goal is to harass your opposing mid player as much and as effectively as you can. By "effectively" I mean not wasting mana at lvl 1 when they're just gonna heal it off, but rather when you're getting close to lvl 6 so you can finish them off with a Riftwalk combo. You want to get them to a point that you can pop clarity and finish them off when they have a false sense of safety with your lack of mana (people generally get too close thinking I can't kill them, but I can :D).
Once you have 1 or 2 kills on the other team's mid, watch the other lanes for gank openings to build up your sword of the occult. This is possibly the most important stage, as most of your attack damage comes from your stacks. This is also why AP items are prioritized as you're building the rest of your main items. Mid-game you're probably gonna be running all over the map like a good solomid should, farming during your hang time from ganking.
Mid to Late-game you have the option of farming the jungle between ganks or roaming with the team. BLUE BUFF IS YOUR FRIEND, GET IT WHENEVER YOU CAN. This build is centered around being mana-efficient, and blue buff speaks for itself. Around lvl 12 you can go grab it if your team doesn't have a dedicated jungler. Even if you do, ask for it politely. :3
Late-game with this build is a little more difficult than with an AP build. AP builds are focused on finding an opening for total nuking and unloading all at once, while this build is more about steady harassment and nuking in small bursts. Your attack speed is great for this since Gunblade gives life steal (not to mention spell vamp) and you'll start to notice the effects of your W right about now as your Clarity's effectiveness begins to wane. Even though this is a hybrid build, you need remember that you're STILL a squishy Kassadin. Life steal helps, but it won't be enough in team fights. During big team fights (where an AP Kass would be unloading and aiming to end the fight all at once), you want to get in, stay long enough to do some decent melee damage along with your spells, and rift right back out when needed. It's also important to remember that mid to late-game you can take down towers fairly well.
-Steady damage output
-Mana efficiency (blue buff+clarity+nether blade)
-More useful in a fight than AP builds once engaged/better chasing potential
-Get some use out of that W (the armor pen is reeeeeally useful)
-Spells don't hit as hard, but they still hit hard enough
-Maintaining stacks can lead to problems
-People might laugh at you once they see your Long Sword
-If you aren't outplaying the other team this build will likely fail (i.e. getting fed)
That's about it, enjoy!