My first Build and i'm quite noobish so be gentle ;)
I will add more info about everything as i learn play better :)

Pros:
High early damage
good at mid
powerfull dot for harashing
Powerfull healing at lvl6
2sec root + slow
Fast to farm creep wave near tower (Nevermove+Flock)
Torment increases all damage including Deathfire Grasp

Cons:
Squishy
You will be targeted on teamfight (coz aoe damage + root + squishy)
Won't get as many killing blows as assists if u have good team
Slowish old man even with +2 shoes
Must predict enemy movements to land perfect Nevermove

Utility tree if something u got to love when u can still get 15MrP from offensive tree.
Utility tree:
Perseverense is nice becose it helps your mana(and health)pool filling up.
Awareness extra exp to lvl up faster... nice...
Meditation is flat mp5 which again helps more at early game(staying longer on lane=gaining more exp/gold, doh).
Utility Mastery is something i personally love a lot. Biggest reason for this is Ravenous Flock + blue buff... do i have to explain... plus u can spam w-e-q more and even few times on raven form without complete oom.
Quickness and Blink of an Eye helps u stay alive, which is goodGOOD.
Blink of an Eye Good if you take Flash. Put this point to Expanded Mind or Haste if you don't.
Intelligence let's u combo more offen (and the more u have CDR the closer w-e-q goes each other).
Presence of the master reduces cd of Flash even further down and also affects on Ignite cd which is nice another dot/health reduction.
Offense tree:
Archmage's Sawy is a nice little boost to our AP. Always wellcome. And Deadliness is just to get next tier(and when creeping early u can crit for 0,66 chance (lol)). If u take Exhaust, then take point on Cripple.
Sorcery same as Intelligence
Archanic Knowledge is biggest reason why we want to dive Offense tree. 15% more than our marks give...

If you are not lvl 30 go first on offensive tree to get Archanic Knowledge and then start filling utility tree.

Greater Quintessence of Magic Penetration and Greater Mark of Magic Penetration for obvious reason: More u have MrP, the more u can deal damage thru at late game.
Greater Glyph of Scaling Ability Power: I like gaining a little more benefit for end game and that's why ap/lvl and not flat :)
Greater Seal of Scaling Mana Regeneration: mp5/lvl help on manaprobs. Per lvl reaches flat as early as lvl 6-7, where u get Ravenous Flock, and after that it a lot better. That's why i don't recommend flat mp5

Flash is squishy champ must imho. It just is lifesaver. Many can argue, but that's what i like :)

Some good Summoner spells next:
Ignite: Nice on early game to burn fleeing enemy champ and heal reduction is gr8 on every phase. Also benefits from Torment. I like using this a lot for early kills (150damage with lvl 1 torment <3) :)
Ghost: Very good when chasing or when fleeing(suprised?). Just remember to put one point from Good Hands to haste when using this. This is other spell i like to use. Also viable choice instead of flash if u dont like/have it.
Exhaust: Some really nice snare to help fighting single enemy or to throw on strongest auto attack hero on team fights. Useful for fleeing also, but if 2+ chasing u find urself thinking "why i don't have ghost instead of exhaust".
Cleanse: Can save your life when u can get out of cc, but i'm not using this spell very often (actually almost never). One reason is that if u get cought on cc, it means they are alredy focusing you and changes to get out alive are slim anyway. Saves lives with flash.


Some not so good choices(but not entirely bad):
Clarity: Could solve sometimes your mana issues on early game and keep u on lane forever, but there's other better choises for this spell slot so i recommend skipping this one.
Teleport: Helps you to get back to lane faster, but is it worth running out of enemy range faster or flashing over wall. But maby if u don't like ignite and rly like this...
Clairvoyance: On full premade groups this could be a valid choice, but i would save this one for some support champ.

Bad choises:
Heal: Maby helps to stay on lane at early game, but not worth summoner spell slot for swain imo (u get better heal after lvl 6 anyway).
Smite: Last time i checked this swine wasn't for jungling... aye... still isn't :)
Revive: Could be nice BUT.... CD.... waaay too long.... not so valid choice...
Rally: What i've heard this isn't such a great for anyone, or at least not for Swain... unless u roll ad swain ;)
Fortify: Leave this for tanks who might get bonuses from it.

First things first. First point on Torment so u can get your harasment up and running. DoT is always really annoying ;)
Second point on Nevermove so u'r harasment annoys enemy even more and from this point on enemy can start failing and let u root+torment first once. After cd's are done u'r laning partner uses something and u throw w-e-ignite and voila... Firs Blood might be your's to claim.
Now starts our damage maxing. Use one point on Decripify and then points on Torment until you get lvl 6 when u get Ravenous Flock. Torment is firstly very neat damage and more importantly increases all damage done by swain, including summoner spells and items. On lvl 3 when u use w-e-q-ignite it means 1,08x(75+75+80+50+20x3)=367.2 and that doesn't even count AP from runes and possible items(ofc enemy also might have magic resistance and u have MrP, but that is just simple math example). And if that is not enough, nevermove's root leaves enemy vulvenerable to your laning partners/gankers abilities and if that doesn't kill enemy it most likely forces him(/her) to at least b.
Ravenous Flock let's u stay in lane and mass murder creeps, but be careful not to burn urself oom.
Skill order that i show in this guide is what i like to take when playing(lover cd of Nevermove a little and increase snare of Decrepify while maxing Torment and Flock), but most important is to take 1 for everything at first and max Ravenous Flock on 6, 11, 16. After u have 1 on everything, everyone must take what feels right and good to play ;)
Also, if u'r not so experienced to throw things on ground, use Decripify to slow enemy so u can Nevermove em.

Harashment:
Throw Torment on enemy as often as u can followed with Decripify if u have it. After lvl 2 try to catch u'r opponent on Nevermove and go throw Torment and on succes. soon u find u'r opponent fearing you and staying out of Nevermove range and out of exp.

Initiate:
Nevermore -> on secces r -> e -> q -> ignite and stay on Ravenous Flock range. Make sure u'r friends know that when root engage all go in. Never initiate alone against more than one enemy and 1v1 consider carefully is u'r opponent fed. U R squishy mage even with flock :D

Endgame group fight:
w -> e -> Deathfire Grasp -> r -> q -> ignite -> try cutting escape of low health enemies with additional Nevermoves followed with torment + lazer bird

Creeping
w -> r -> if tank/super minions e -> q

Blue buff/baron/dragon:
w -> e -> q -> r -> delay (depending mana and situation) -> r(flock off) -> wait all cd to rdy -> w-e-q-r

Always:
DON'T FORGET TO TURN Ravenous Flock OFF WHEN NOT NEEDED. You will oom...

Next some good items. I add explanations later.
Deathfire Grasp
Sorcerer's Shoes
Spirit Visage
Abyssal Scepter
Will of the anchientsWill of the anchients
Rabadon's deathcap
Archangel's staff
Rylai's crystal Scepter
Hextech Gunblade: if u have much money go ahead and buy for sixth item. Still quite over expensive for Swain. Torment+Deathfire Grast+Hextech Gunblade+Decripify=gg

Very often i find myself starting with Amplifying Tome + Health Potion, but when taking solo or mid Doran's Ring is my choice.
Early game:
Kage's lucky Pick or if u'r forced early to go b and u started with Doran's, u can take Meki Pendant or pick the tome now.
Sorcerer's Shoes
Tear of the Goddess

Mid Game:
Deathfire Grasp
Needlessly Large Rod
Spirit Visage for Defensive item. health+magic res+ 15% more healing lesser ravens (+ phossible WotA)

End Game:
Abyssal Mask or if some1 in grp have it i often buy Will of the anchients
Rabadon's Deathcap
Archangel's Staff