I started playing swain a while ago but i started again and i have to say i love him this is my 2nd guide so i hope its good :)
just make sure you go mid with swain as this is what this guide/build is based on

pretty flexible when it comes to items, but as long as you end up with this at the end you will be fine take dorans ring at the start for early ap and good mana regen as well as health dont want to be to squishy. The archangels staffs will give you more than 498 AP in game as from the bonus you get, you should be able to get up aroung 800-900 all being well

Pros-
Decent farmer late game
Really powerful early/late game
Good get away (ult)
Really fun
Dominates lanes
Cons-
Squishy early game
Weak farmer early game

Masteries are really handy to have early game if you gonna change them just make sure you keep with mana regen/flat mana and AP

Havent completed my rune page yet but this is what it will look like hope this works its really good to have cooldown reduction as spamming torment and nevermove will tear the enemy team apart

now i cant stress this enough ALWAYS ALWAYS MAX torment first it has the best dmg and finishes your enemies off really quite well. But if you max all 3 abilities first then you can combo, lazer bird (decripfy) so you can sloww and land your nevermove then torment then ult when you get it a few auto attacks and they will B straight away. When your running from a fight make sure to ult and nevermove to stop them and heal it can help get you away really easily

up to you with these but flash is great clarity is really useful up to about lvl 15 but if you want take ignite for more finish or exhaust/ghost to slow catch up or get away

Make sure you last hit with swain not the strongest farmer till he gets nevermove and ult so just focus last hits and you will do fine

All in all swain is a really good champ to play as, fun and powerful buy him now while he is half price!
enjoy summoners and i hope to see you on the fields of justice