Hi. About Swain, he is fun! People underestimate him, his power and his survivability. So, I have made this runepage for Swain (already had it like this for mage, but its effective with Swain).

Why Armor?
Armor is effective early game, but not against mage. So that's a choice you can make, armor OR magic resist,, or both. Anyway: if full build, you got some pro armor which automatically reduces the DMG from ppl like Xin or Caitlyn.

Why MP and AP?
I don't think I need to tell you why you need magic penetration and ability power... It's a mage char! You could also use CDR in Seal.. but that's user defined.

For the items, people think: "Mercury Threads,,, U MAD?". Nope, I'm not. Like I said: you'll be the semi tank of your team, the 'if the real tank dies' tank. I will explain later how to!

Start with the Doran's ring. Buy Boots of Speed. Get the Mercury threads and start building your Catalyst the Protector for the Rod of Ages. In the meanwhile you will get some kills, pwn a Tristana and will the time allow you to buy all 3 items for the Rylai's Crystal Scepter.
Before you get it, build the Rod of Ages. And afterwards (need something like 750G) get the Rylai's. At finish get the Mejai's. Like said again, you'll be the reserve tank. That's why you shouldn't rush the first item.

For some more action, healing and DMG with your ulti, stun and Angry Bird > Rabadons. The Morello's Evil Tome is just for CDR.

You shouldn't play Swain as a full tank. You don't have the proper items, runes and masteries for being a 100% tank! That's why you should walk in 3th row.

you and Carry
DPS and Support
Tank

Easy as that. If your team engages, place your stun on the right spot (obviously XD) and get in with your ulti! Mana gone? CLARITYYYYYYY And pown.

Never push too hard with Swain (solo or with mate) always make sure you have at least 2 mates near (like Xin with E spell or guys with flash). You are not able to get double kills 1v2 with just your ulti, stun and these items.
Ignite?? Yea, I know. It's kinda useless when you play proper.. Ingame I have had no kills yet (out of 5 games). Just make it do DMG when they'r low HP, ulti, stun: BOOOM.
GG

If you need more help with Build or questions about how 2 play with Swain properly, add me ingame (NRG 1mRAAAH) or go to our team forum @ nrg-esports.tk!

Thanks for reading, watching and rate!!