Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
This is not stupid op carry as a support, this is Swain played as a support in a support role. Yes, you can build him for massive damage, but that is not what this guide teaches. This guide is about being the biggest douchiest most obnoxious hardest to kill king of cc to ever wander botlane.
Your role as Swain support boils down to two things. Tank, and CC. As a tank, your goal is to constantly be in the way of the enemy, and to simply not die. easier said than done, but with your root and slow, you are pretty good at escaping alive.
In terms of CC, you actually have better than many supports. In fact, with the right build, you have more. The two key items for this are your relic shield progression which gives you a small heal, and rylais which turns your ult into a never ending AoE slow. Your root is very effective for zoning even behind enemy minion waves, and your slow is very useful for poking, and setting up your root.
Exhaust is almost essential for swain. As a support, not just slowing a fleeing enemy but also for mitigating damage from the enemy can turn an encounter in your favor. It also helps in mid to late game for keeping key targets within your ult. You can swap flash for ghost if you want the speed burst for your ult/slow/snare, but I prefer the excape opportunities provided by flash.
Items should vary based on enemy team comp, skill, and playstyle. This is my generic build that I base my modifications off of. Remember when building, your NOT mid-Swain. You are a tank support setting up your allies with some easy kills. Too much ap, and you may find yourself KSing, or taking farm unintentionally.