First, let me state that this is NOT a build for ranked. I have not used this build in ranked because I have only played one ranked game and I just don't like ranked. More power to you if you like ranked. I just don't.
Second, the item build, as all item builds are, is purely situational. Feel free to get different items if you feel that you need something else.
This is my second(and better) build for Swain, The Master Tactician. AkA: Jericho Swain. *evil grin* I hope you're ready to be the best AP carry in the game. ;)
The AP stat on this page is wrong. With the full build Swain has just under 700 AP.
Also, this is a guide best used for mid, so at least try to be your teams "mid".
Let's begin.

PROS

-Easily dominate early and mid game.
-Walk into a 3v1, get a triple kill, and walk away with full health(I tend to dance after that. :P)
-DOTS DOTS and more DOTS... You can't go wrong. :)
-Can auto for over 1k(with Lich Bane of course :P)
-Harass and cause terror throughout the course of the match.
-Your team will love you while you piss of the enemy("ragers" are common against Swain, as are "trolls")

-I am a pro with the snare and I hit with it 95% of the time, so if you face me in game, expect to surrender. ;)

CONS

-The main focus of team fights.
-Hard to get the hang of

Nevermove placement.
-Just plain powerful... Oh wait, that's not a con. :P

I go with straight AP/lvl runes. You can go with straight AP runes if you like for more early game damage, but Swain does enough damage to begin with, so you probably won't need it.

I go with a 10/0/20 mastery tree, placing points for magic pen., ignite, clarity, and cdr.
It's just what works for me, but you can change them if something else works better.

This is where you get to laugh hysterically at your opponents.
Start with theamplifying tome and a. Most of the time I stay in lane until I hit 13 before I either die, or need to "b".
On your first "b" you should have enough gold for theHextech Revolver, if not theWill Of The Ancients. If you can only get the Revolver, get yourSorc. Shoes before upgrading the Revolver. After you get the spell vamp. book(), getRabadon's Deathcap.
After that, get twoHextech Gunblades, then another Deathcap().

The core of your build, being those that are most needed, are Will Of The Ancients, Sorc. Shoes, and Rabadon's Deathcap. All other items can be changed to situational need, and preference. As I have said before, this is just what works for me.

Start withTorment, and this will be your "first to 5". It does a lot of damage over time(DOTS) and makes you do more damage with everything.
Next to 5 will beNevermove. This is your snare, and will probably be the main "first spell" of your combo.
last is your slow,Decrepify. It does the least amount of damage, and is really only good for it's slowing properties.
Of course you'll be putting points intoRavenous Flock whenever possible.

I go withIgnite for the extra Ability Power, plus it's another DOTS, and that's always welcome in my opinion.
And I also getClarity. Now I know someone is going to say, "Clarity is stupid to get with Swain. His passive restores mana." To that, I only have one response. It's not exactly effective, being that he needs to KILL a unit for it to activate, and I go through mana faster than minions spawn, and it makes that early game kill easier to obtain.
If you don't like the summoner spells I use, then feel free to use other ones, just change the mastery pages to suit the spells you are going to use.

When using ignite, start with ignite, then Torment as soon as you see your opponent. Most will be between half and two-thirds health after this, and after another Torment, you should be lvl 2, and have Nevermove ready. Snare with Nevermove, then Torment and auto and you should have your first kill, or you'll have pushed them to go "b" and you'll hit 3 or 4 before they get back. Use your health pot. when you feel the need.

Snare with, approach and, activateRavenous Flock, thenDecrepify, and if they're not dead yet, hit them with anotherTorment.

Most of you are not going to like this build, or this guide, and are going to have some sort of criticism, but this is how I build Swain. If you don't like it, don't use it. If it doesn't work for you, try something else. If you've encountered me in game as Swain, then you know it works for me. I must ask however, that before you say, "This build sucks." Or you criticize my build, just remember that I told you that it won't work for everyone. Try it anyway. It might work for you.