Alright, well, I like playing with Swain, but I hadn't ever really found a build that I consistently liked using with him. With the release of 3 new items in the recent patch, I found one that sparked my interest and led me to believe that it was a viable item to build a new Swain around. Said item...Eleisa's Miracle (unfortunately, it does not appear to be in the Mobafire database yet and is not yet listed above...)

Now, to be clear, this is a support tank kind of build. It has proven to be a nasty defensive build in my experience. You have enough mana regen to stay in your ult for a long long time, and you leech enough health in ult to stay alive through some pretty wicked damage. I have been able to hold a lane against 3 champs pretty easily and have been able to put up a pretty stingy defense against all 5.

Gonna put this here as it is the most important thing about the build.

GET THE BLUE BUFF. KEEP IT. Once you hit level 11 is ridiculously easy to solo the golem, so have the blue buff whenever possible. Go steal theirs if you have to. Impress on your team that this is crucial to your build. Most will know that anyway.

Not gonna lie, this is expensive and takes a while to get offensive with, but you will have survivability in lane very early on.

Your main goal early is to get your mana regen items built.
1) You want to start with Meki and move to your tear as quick as possible.
2) Then I like to go with Fiendish Codex to get a little AP with my regen (CDR is a plus here a well).
3) Build your Philosopher's Stone and upgrade it to Eleisa's Miracle ASAP.
*somewhere in here I build my boots of speed, but as usual, that's a very relative thing.

Next, you want to stack some more survivability.
4) Spirit Visage has synergy with your ult, so it is a no brainer with is bonus health and CDR.
5) This is your situational defense spot. If they are an AD heavy team, I really like to get Sunfire Cape here to give extra help and damage when I wade into the fray as a big bird. However, Force of Nature is also a great choice. I don't have much fear of either dying or being CCed in this build, but if you are the kinda player that really likes Banshee's Veil or Guardian Angel, those would e appropriate here as well. If you wanted to add extra offense instead (not recommended) Rabadon's Deathcap or the Void Staff (which I would take, if I was going to) would be where to look. Repeating: I recommend a defensive item be your final item.
*Sorcerer's Shoes are what I usually take, but I've been considering experimenting with Boots of Swiftness instead.

After you've finished this part of the build, the icing on the cake is finishing Deathfire Grasp and Arhangel's Staff for AP and finally some legit damage. I cannot stress the importance of DFG. It gives you the burst ability to get some of your own kills late game.

Ghost and Flash
Both provide and escape when you need one. They have kept me alive with this build when I'm not paying attention, run out of mana, and need to beat a hasty retreat.

Outside of those, Teleport is probably the only other viable option.

Start with Decrepify for harassment purposes early. Then get Torment for damage and Nevermore for stun. After that, it's max Torment, Decrepify, Nevermore in that order putting points into Ravenous Flock whenever possible.

I could see an argument for maxing Nevermore before Decrepify, but I like being able to deal some damage with my laser-bird, so I've never tried it.

Pretty straight forward.

21 in D tree. We are playing him like a tank after all.
9 in Support. This will give you some regen, some mana pool, and make your BLUE BUFF last longer.

Mana regen is the name of the game, and you can never have too much mana.

Don't forget to GET THE BLUE BUFF.

I think the best term for this build is a Situational Tank. You are not a full on tank, but you can take lots of damage and harass in team fights like crazy. You won't generate double digit kills with this build (5 or so is not out of the question), but you will have very few deaths and a ton of assists. That denies the other team xp and gold.

This build is an excellent distractor in a team fight and can wipe out a creep wave in no time flat. I know I sold this as a defensive build, but realize that you are also excellent at leading the charge to wipe out a tower. You can decimate their creeps and give your own a clear path to destruction.

Paired with a DPS or burst champ, you are a huge help to their killing (stun them and drain them, then let your partner mop up the leftovers).

Try it out, it's fun. I came up with it yesterday and played around with it last night, so there is probably lots of room for improvement. I look forward to comments and suggestions. Swain needs more love, and maybe this build can give it to him!