This is a support build for Swain. This build helps Swain generate gold early to bulk up to a beefy support who can stay in a fray for a long duration by mid game. Late game he supplys the team with offensively oriented auras and locks down opponents with snares and slows. He also boasts the infamous tormented deathfire combo to help the team take down the first kill target effectively. I would classify this Swain to be a bulky AP hybrid.

Pros:

Improved Farmer
Great End Game Damage (AoE and Single target)
Great End Game Survivability
Great Support
Gets to End Game Quickly

Cons:

Squishy until mid game
Low single target out-put until mid game
Heavily focused until late game

For Swain I choose two summoner skills with a lot of utility; Exhaust and Flash. These prove to be versitile and help Swain all game while his role and playstyle will shift through the game's progression. Allow me to explain.

Flash: The summoner skill we all love...and hate. We will be using Flash mostly defensively for the first half of the game and mostly offensively in the late game.

Flash will allow us to distance ourselves from focus and ganks early on and allow us to get into the fray to slow and snare opponents late game (and to get good use out of lazor bird, sunfire cloak, and ravenous flock).

The same idea for exhaust. We will be using exhaust to keep ourselves and friendlies alive early on and later we will use it to further slow, debuff, and support in team fights.

This Swain is a bulky AP support. Due to the nature of supports, they tend to be heavily focused in team engagements. Since this is true, we want our Swain to have some meat to him without sacrificing too much of his damage out-put so he can still effectively farm. To do this, Swain needs defensive masteries. Grabbing magic resis, and armor will make up for the lacking of said stats in the itemization and give Swain increased survivability in the early game when he is most vulnerable.

Evasion and nimbleness will amplify the effects of dodge runes (dicussed below) and will give Swain increased mobility. Extra hp is always nice, Aldor is a great skill of course and 4% reduced damage taken will help you stay in frays.

Extra ability power, reduced cds, and extra spell pen will help keep Swain alive through spell vamp, help farm, and help him support as well. Cripple will give additional spell pen and increase the duration of exhaust to keep teammates safe and help kill focus targets.

Swain's scaling with AP is not terribly impressive. To compensate for this we are stacking magic pen marks and quints.
Itemization is slow to cover magic res and armor. because of this we are taking mag resis glyphs and dodge seals.

The seals will help with nimbleness to increase Swains movement speed to out run enemies in the begining and close on them late game.

Our Swain is going to start very very squishy in team fights due to getting gold generating items first. Because we will be doing this, we will want to stay far away from the enemy and yet still support the team. To do this we will level Nevermove first. This AoE snare will help peel friendlies and gank focus enemies caught in its effect. Leveling this ability first will also allow Swain to farm effectively when used in combination with Ravenous Flock.

Our secondary skill to level is Decrepify. The slowing effect of this skill will help Swain peel enemies of himself and friendlies and will help him land nevermove. The damage scaling on this ability is also naturally good.

Torment is our last ability to level. This is uncommon in a Swain build but I'm not sure why. Let's face the facts that 20% increased damage for a character that really doesn't have early game out-put is not worth leveling first. Late game, the debuff this provides will be amazing when used i ncombination with a Deathfire's grasp and a sunfire cloak (along with your other abilities) but early on Swain is too squishy or does not do enough single target damage to utilize the debuff effectively. To attempt to chain his abilities in a predictable fashion as necessary to try is to leave yourself wide open to get ganked (you will have no cd's to protect yourself).

Ravenous Flock is, of course, one of the major reasons anyone would choose to play Swain. It is great for survivability and for farming and should be kept at max level available.

Swain's damage scaling with AP is not terribly impressive. Due to this, the champion is often overlooked and underplayed. Swain does, however, have a fantastic skill set and a great passive. To get the most out of the skill set and to cover inherent lackings in the overall character, a unique and creative itemization is chosen.

Getting the most out of your gold per 5 items is essential for Swain as he is not a fantastic early game farmer. He also runs into early game mana issues and lacks survivability until level 6. To compensate for these weeknesses careful earlygame play and the right itemization is neccessary.

I start my Swain with a regrowth pendant. This gives Swain a lot of resistance to early game harass. It will allow you to take early game harass for your partner so the two of you can score lane advantage. Pick your battles wisely and if possible lane with a stun as this will help you land Nevermove more easily. It is important to focus on the gold farm while laning. Farming gold and harassing is possible due to nevermove's aoe. Utilize opportunities to harass and farm with a single spell as much as possible. Last hit creeps to get gold and to get mana return. If you fail to do this you will be highly disadvantaged in the lane and crippled for the rest of the game.

As soon as you farm enough gold go back and buy a philo stone. The gold/5 is great, the health regen is nice to mitigate harass and the mana regen will allow you to now harass and farm at the same time.

Farm the lane until you have enough gold for a Kage's and boots of speed. Then return to the shop to purchase these items. You will now notice that you farm gold very rapidly.

I like to go back to the lane at this point and farm more until I can both get Sorc Shoes and a giant's belt. It is still a bit soon for team fights to begin, but participate in them (conservatively) if they do. Since you have a lot of gold/5 you don't have to be in a lane farming at this point to advance your gear. Do be careful, however, because you are very squishy at this point and easily focused. You can however take some casual harass as your pendent will regen keeping you with a full hp bar. Avoid crowd control at all cost while supporting your side with slows and snares. Don't waste mana on torment unless the enemy is cc'd or overextended and you are sure you can land all your abilities on them. Complete a sunfire cloak asap.

Once you obtain your sunfire cloak you are gold generating machine. the sunfire damage will keep your mana near full and your pockets heavy. I like to get the golem buff at this point so that I can walk around with my ult going spamming nevermove on every creep wave. If you can get to this point without your team falling behind you are on the fast track to success.

When you can pick up a Rylai's for additional slowing. This will keep you alive via kiting, keep enemies from escaping you and will give your team huge mobility advantages in team fights. Landing nevermove after obtaining this item is a trivial task. You are now beefy enough to go toe to toe with enemy carries and should be in the fray in team engagements.

Next buy a deathsfire grasp. Now your torment become's feared. By adding this you have an arsenal of single target destruction as well as AoE support damage. Torment>Deathsfire>Decrepify>Nevermove>/dance as your sunfire and Ravenous Flock finishes him off.

My next purchase is usually an Abyssal Scepter. I sometimes substitute a Force of Nature in here if I'm having trouble catching, surviving heavy magic, or if my teammates are mostly AD based. The debuff is a noticable team dps increase, especially on carries, and the MR increase is a noticable increase to personal survivability. (If the team is heavy on CC don't forget about the Banshee's Veil it is great against annoying stuns, snares, and taunts)

My final item is likewise a tossup. I like Shurelya's Reverie if my team is using a blitzing stratgy. I enjoy Will of the Ancients for more sustained fights. Over all it depends on your team's make up (and their's as well) and also on how much you are being focused. (I go with an Abyssal Scepter usually if I go with a Force of Nature or Veil the item before.)

Swain is a difficult character to build because of his lack of focus and touch and go survivability.

I feel that this build covers inherent early game weeknesses and slingshots the character to late game through careful selection of items, and abilities.

Though slow going at first this Swain boasts huge supporting and damaging capabilities in the late game while mantaining excellent survivability.

I hope you will give my Swain a try. Make sure to check out my Alistar and Cho'gath builds as well!