The reason I made this guide was to create a Swain that could stay alive during team fights without losing all his mana to Ravenous Flock. I also noticed that some other guides would need you to get the blue golems in able to survive team fights and clear minion waves. So... this guide was made so that you wouldn't need to get blue golems to spam Ravenous Flock all while surviving team fights. Warning: Expect to get a lot of assists and Have fun!

EDIT: THIS GUIDE WAS MADE SO THAT I COULD LOOK BACK ON IT WHENEVER I PLAY AS SWAIN. I WASN'T TRYING TO COPY OR STEAL ANYONE'S IDEAS BUT INSTEAD MAKE A GUIDE THAT WORKED FOR ME SO I COULD USE IT ANYTIME I WANT TO.

Greater Mark of Insight is used to do damage early game. I use it for getting kills which helps build late game items. I have never seen what it does late game because my team either wins or loses.
Greater Seal of Clarity helps a lot with mana regen late game so that you can keep spamming Ravenous Flock.
Greater Glyph of Shielding is good for surviving team fights since half the time you'll get focused which reminds me! Warning: You WILL get focused most of the time.
Greater Quintessence of Fortitude is another survivability rune.

These are very standard masteries that most people use for Swain. Most of the guides I've looked at use these so I decided I should too.

Sapphire Crystal is a good item for staying in lane if you solo. You can hold people off of towers or get them killed when they're near towers by using Nevermove. The two health potions backs up your solo laning by keeping you alive for as long as possible before teleporting back.

Sorcerer's Shoes give you magic penetration to help with those early kills! Or to help get rid of those pesky minions.

Catalyst the Protecter is used to get one of your core items. Hint: Core items are Sorcerer's Shoes and Rod of Ages.

Building a Rod of Ages allows you to get a lot of mana and health for once again... survivability. You can also do more damage to possibly get a kill!

Archangel's Staff has the main purpose of giving and regenerating mana so that you can use your Ravenous Flock for what feels like whatever. I find this item to be one of the most useful since your Ravenous Flock is where most of your damage comes from. But then again your kills may get stolen since Swain isn't really a damage of time or burst type of character.

Will of the Ancients is a key item to surviving during team fights and tanking all the damage focused onto you. It allows your Ravenous Flock, which already gives you health back, to give you even more health back!

From here is usually how far I get before the game ends. But depending on how your game is going, you can choose Rhabadon's Deathcap for more Ability Power, Abyssal Scepter for Magic Resist + Ability Power, or Zhonya's Hourglass for Armor + Attack Power.

I find that upgrading Torment first is the best because of the damage it does while you're attacking your enemy. Decrepify is upgraded second to multiply that damage. Nevermove is only upgraded once because the snare has a set time no matter the level. I usually use Nevermove just to stun the person long enough so that me or my teammates can kill them. Ravenous Flock is self explainable because it's your main spell of damage! Upgrade this whenever you can.

Teleport is used to quickly get back to your solo lanes or help your teammates out in the fastest possible way. Flash is used so that you can escape from ganks, dives, and team fights.

Use Decrepify to slow down your enemy and immediately cast Torment to do as much damage possible. Aim Nevermove right afterwards to snare your target and then Ravenous Flock to finish or damage them.

In conclusion this is the build that I use. It may not work for other people. If this build did not help then I'm sorry for wasting your time. Most of the time your losses will be caused by your teammates unless you feed. But anyone you do feed, you can kill later late game. This is a late game build that will give you a lot of assists.