Many people play swain the same way, a great AP nuker that is dangerous when in his ult form. There is one minority, a huge one, about the build. You are squishy, period.(mostly early game)
This build shows you how I play swain as a Mage focussed on HP early game.

This build is made AFTER swain got his passive nerfed in a terrible way and since i hardley build mana these are my runes.

Red : Mana per lvl
Yellow : Mana regen per lvl
Blue : Magic pen
Quints : 1 mana regen per lvl
1 Ability power per lvl
1 Flat Abillity power

There are two ways to succeed this build
9/0/21
Or
9/21/0

The defensive builds suits the entire idea behind my tanklike build, but, because the passive got nerfed and I hardly focus my mana I suggest you go 9/0/21

Taking 2 points in buff duration and 1 in improved flash.

I claim to get you godlike with only 38 AP though my build has plenty of AP.
So buy the items I tell you but as soon as you get to soulstealer buy every HP/mana item before upgrading them with AP for instance buy

Giant belt and 2 ruby crystals before making rely and banshee

AND please start with Doran ring + HP pot, i didn;t put this in the build to give correct end game numbers in the "cheat sheet."

For summoner spells i take flash and ignite
clarity is good too cause of the lack of mana but I rather have some escape ability.

So you might wanna give up ignite for clarity but I rather have 1 extra DOT (damage over time)

Laning mid should be your priority, don't fight over mid though keep the game fun!
ofc when you have a karthus in your team or someone else with an important ult you let them mid.

Why mid? once you got the hang on mid, kill the other person once or twice, you can;t lose it anymore. it's already too late and you will feed on him like crazy, making you a viable carry.

the combo is simple, rely on your E it does massive damage, is easy to land and makes all other abilities strong. harass with E not with Q like many swains do, Q is easy to walk out from. Also when under the affect of E auto attack the champ, it does some nice extra dmg.

When you get lvl 3 it's time to party. Land a successful W and you win, period.
Every time your W seems to be succesfull, cast E and Q for some massive DOT damage.my experience is about 40% of the hp will be removed by that combo. that's all there is to it actually... snare, dot and slow on and on you extra HP will make you go onto melee range with no trouble since swain had little to no range. you will be godlike before you know it!

and for those who have trouble playing vs kat (as i read on the forums)kat is the most easy target for swain, just press W at your location every-time she wants to shunpoo on you and you got her where you want her.

PS: i laned vs tanks like morde and cho too with this buils, i made them seem as resistant as ezreal is... man did they get owned.
I only have trouble vs lux, vlad and sometimes corcky, ashe, tris

the combo is the same try reading through mid lane section to "learn" the combo. but your partner is most important. get yourself someone with a stun, snare or epic slow. this makes you W easy to land, and your combo easy to apply. a laning combo like that is actually very OP for instance, SION + SWAIN, TARIC + SWAIN etc.

and for the extra fun, try laning with a singed some time, singed + swain is the best laning combo i ever experienced overall.
You snare with W, giving him an easy fling INSIDE his poison, you apply E and Q and he has many DOTS upon him and 1 or 2 slows if singed slows too... that is some serious fun!

so have fun guys and enjoy the build! comment anything you'd like