Not Updated For Current Season
So, this is my first build I've posted on here. I've been using this build a lot recently, and seem to do relatively well when compared to other Swain's I encounter, so I thought I should share.
The selected Masteries are also a straight forward choice, and common for most AP characters.
There's some debate as to how heavily, and if you should, put points into Defense. For my play style I like it like this as it gives me far greater Utility throughout the game, as well as more damage.
I do have a Mastery page that weighs heavier into Defense and I use that if my team has selected no true tanks, but recently my teams have all had traditional tanks, so I lean more towards AP/Utility.
The runes are fairly straight forward, playing Swain with an AP slant.
Greater Quintessence of Magic Penetration I consider these a must. I've seen people opt for various other Quintessences, but these make it so you very rarely have to worry about Magic Penetration during games after you buy your Sorcerer's Shoes.
Greater Mark of Magic Penetration are also a must. Nothing really comes close.
I personally choose 9 Greater Seal of Replenishment, but some people may also choose Greater Seal of Scaling Mana Regeneration, or a mix of both. This decision will be based on whether you like that mana regen boost during early or late game. I find that once you have the Blue Golem buff these Runes become a moot, so keep that in mind. Further, Swain's Carrion Renewal passive makes you regenerate mana every time you get a kill shot. If you're confident in your early game last hitting ability you should really consider Greater Seal of Armor - especially if you're playing with more of an AP/Tank slant.
For Glyph's I run a mixture of early and late game beneficial Cooldown Reductions in the form of Greater Glyph of Cooldown Reduction and Greater Glyph of Scaling Cooldown Reduction. You may also consider Greater Glyph of Ability Power if you want to go for an early game kill.
Flash is a must. It's great for getting you out of tight spots, or into the action. It's also excellent for chasing.
For my second spell I run Ignite. It can be crucial for fights against fellow Swains, or any other character that relies on self healing. It may also get you a kill, which is always a plus.
Other spells to consider are Ghost and/or Teleport, but that comes down to personal preference in my opinion.
I've seen some people run Clarity, but I don't feel that should be necessary in most situations.
I didn't post the full item list including lower tier item parts because it's pretty easy to follow on Swain. The items listed give you a heavy AP slant, with some Spell Vamp and Health.
Recently, when going solo in lane, I haven't been using the Philosopher's Stone, and have been rushing a Rabadon's Deathcap after I finish my Tear of the Goddess, Catalyst the Protector, and Sorcerer's Shoes. I'll then build my Rod of Ages and finish the Archangel's Staff. I've been finding this gives me a good early game advantage in my lane, when paired with the great staying power of the Carrion Renewal passive and Ravenous Flock ultimate.
Rabadon's Deathcap is a standard AP item, used by almost all AP builds due to its amazing passive.
Rod of Ages will give you a substantial amount of HP/MP, AP, and an excellent passive effect during early and mid game. Tear of the Goddess is a good item to buy early as the passive effect carries over when you upgrade it to an Archangel's Staff, and it can take a while to cap out otherwise. Archangel's Staff will give you a huge AP boost, which will sometimes be helped further by the purchase of a Frozen Heart as a sixth item (this statement is explained in a couple of paragraphs). These three items work well on their own, but do incredibly well on Swain when used together.
Will of the Ancients is a very nice item, and can have a massive effect when you use Nevermove and catch a number of targets.
I tend to sell my Philosopher's Stone when I'm working on my sixth item. I have my sixth item listed as Frozen Heart, but it's a somewhat situational slot for me. I will sometimes pick a Spirit Visage if I'm surviving team fights, but only by a little bit. The additional HP, magic resist, cooldown reduction, and healing effect on Ravenous Flock is noticeable, and saves lives!
Other items to consider in the sixth, and sometimes fifth, slot(s) would be Thornmail for heavy AD teams, Banshee's Veil against a heavy AP team, etc. The items I listed are only intended to function as a guide and you should really try out different things and see what you like.
A lot of guides will tell you to buy Rylai's Crystal Scepter. I don't like this item on Swain, for the simple fact Swain already has a slow effect from Decrepify. That doesn't mean you should never use it. It gives a massive HP boost, as well as one of the largest chunks of AP. If you're playing against a team that is crushing you in seconds it's certainly worth considering.
This isn't a set in stone sequence, but it's a pretty reliable order.
Open with a Torment to start your DOT's, and get a damage increase for the next few seconds. Follow up with Decrepify to make use of the Slow effect, followed by Nevermore to snare the target.
When you've got Ravenous Flock you will find it useful for almost all engagements, especially so if you're surrounded by multiple targets where you can gain the most benefit from the health drain effect.
I like maximizing Torment or Nevermove first, depending on which I'm finding the most benefit from. Nevermove can provide some amazing early game farming ability, while Torment can melt enemy health in a one-on-one. Get an early point in Decrepify to gain the slow effect and add to your damage output, but I wouldn't maximize it until later. Remember to put points into Ravenous Flock every time it's up (6/11/16), as it's your most important ability.
Just to make sure it's mentioned, your passive is Carrion Renewal. It will grant you mana (based on your level) following kill shots of minions and champions. This is incredibly powerful early game as it allows you to stay in lane almost infinitely if you play smart, and late game when paired with Blue Golem buff will allow you to keep your Ravenous Flock active for extensive lengths of time.
The primary roles of a tank are to draw the oppositions focus, take the brunt of the damage - specifically the initial burst, and to disrupt the oppositions flow. Swain can excel at all three of these roles if used correctly.
You're going to want to keep your 3 Greater Quintessence of Magic Penetration and 9 Greater Mark of Magic Penetration, but change over to 9 Greater Seal of Armor and 9 Greater Glyph of Magic Resist (or Greater Glyph of Scaling Magic Resist, depending on preference).
For your masteries you should build something along the lines of 0/21/9, although there are a number of variations that people use. I've seen 9/21/0 work before, as well as a 9/12/9 hybrid.
For your spells you may like to run Flash and Ghost or Teleport to give you greater maneuverability around the map and in/out of team fights.
You'll still want an initial item build of Philosopher's Stone, followed by shoes (explained later), then Rabadon's Deathcap, but this is where it changes.
If you're against a CC or AP heavy team, I recommend getting Mercury's Treads over Sorcerer's Shoes. You can build a Banshee's Veil where you'd normally build a Rod of Ages. I'd then build an Abyssal Scepter, to make up for your lost Magic Penetration and to boost your Magic Resist further, and then finish off with a Spirit Visage. At this point you can sell your Philosopher's Stone and pick a situational item. You probably won't need much more Magic Resist that you already have, so you might choose a HP ( Rod of Ages?) or Defense ( Frozen Heart?) item at this point. Both of these mentioned give you some survivability, and a little more offense.
If the opposition don't have many CC or AP players, you could opt for Sorcerer's Shoes over Mercury's Treads, skip the Abyssal Scepter, and build your Philosopher's Stone into a Eleisa's Miracle. While you'd still get a Rabadon's Deathcap as your third or fourth item, you can then be a little more versatile with your fifth and sixth slot choices. I'd still recommend looking at a Spirit Visage here, due to its excellent stats and relatively low cost.
Will of the Ancients is a good item in both of the above mentioned builds, but it's a little harder to fit in here. Your damage output will be lower in a tank role, and so your Spell Vamp will see lower returns. That said, keeping yourself alive that little bit longer can be crucial and it will really come down to what you think will benefit you most in each specific game.
Either way, you're going to be putting out less damage than a straight AP build with this setup, but your survivability goes through the roof. If your team members didn't pick a tank during selection you can fill a crucial initiation/tank role, while keeping your damage fairly high and causing the opposition trouble.
I'm still working on this guide, giving it frequent updates. Please feel free to comment and let me know what you think, and of any improvements you think I can make!