Not Updated For Current Season
This is the best build for Swain. Period.
Grab a doran's and a health potion beginning of the game to give you an edge over your enemies. After you have acquired some gold, head back and buy boots of speed, since Swain is quite lethargic. Go get some kills and farm a bit, and then progress towards building a tear of the goddess and a blasting wand. After this, buy some sorcerer's shoes to give you more mobility and some magic penetration. Now here comes the fun part. As you can see, the goal is to get as many archangel's staffs as you can before the other team either surrenders or gets obliterated.
Here's why you want to stack archangel's with Swain:
For starters, in general, Swain needs as much AP, mana, and mana regen as possible. An archangel's staff provides all of these. At first glance you may be wondering how the hell a bunch of archangel's will make you an AP monster. The trick is in the passive and unique passive. The passive is that 3% of max mana is converted into AP, which is awesome. It becomes even more awesome when you have six archangel's and that becomes 18%. The unique passive is that each time you use an ability, your max mana will increase by 4 (keep in mind that the effect has a 3 sec cooldown).
Here comes the grand finale. :)
At level 18, if you are not a noobasaurous, you should end up with:
-Approx. 1,000 AP
-Approx. 4,200 Mana
-Depending on # of archangel's, possible 150 mana per 5 sec.
As you know Swain is a bit squishy (meaning he doesn't have a lot of health) <-- for noobs
This is taken care of in the beginning with the doran's and health potion, and then with the addition of more and more archangel's staffs, his ultimate takes over. With the enormous amount of mana that you will have, plus the massive mana regen, his ultimate will keep you alive all day. With that much AP, his ultimate will do loads of damage and you get half of that back in health. With the mana and mana regen, plus swain's passive (carrion renewal), it takes a VERY long time for his ultimate to run out of juice. If you still don't believe me, consider this: With six archangel's staffs and the stats that I mentioned, I have, in game, TAKEN DOWN BARON BY MYSELF. Conclusion: You are not squishy anymore lol.
As for the runes and masteries, they explain themselves. As far as the skill sequence goes, start out with decrepify to do some harassing, then add nevermove so you can trap them. After that, you want to level up your torment skill as quickly as possibe, since it is the most important. In the end though, it is Swain's ultimate, ravenous flock, that will keep you alive and piss off your enemies time after time.
With the outrageous AP, and seemingly endless mana, this build will give you what you need to unleash "relentless rAPe" upon the other team.
Go kick some ***. :)