This is a great swain build for solo q if you feel like solo carrying and is confident in your ability to land your root. Its inspired by quas, a swainmain in challenger so its legit really good. Im a swain main and follow this thought process every game. Climbed from gold 5 to plat 5 playing only swain using this build. It took 160 games and is my first time climbing. Worked wonders and everytime i won lane, i won the game.
Starter item:Bellow plat dorans ring, plat and above dorans shield but get a feel for it cause they are good in different situations.
Core: First back usually blasting wand and ruby crystall if you want kill preassure, blasting wand and darkseal if you got a kill on a medium squishy before your first back, catalyst if you solely want to farm until you get tower, ninja tabi rush if auto attacking ad laner. If you face akali buy null magic mantle. After this decision build roa. Then you build ordinary boots. Then if you face an laner scaling with ad you build seekers armguard, if not build the book. Then finish zhonyas and if you did not rush ninja tabi you build sorceress shoes.
Fourth item: If you are behind build liandrys, if there are two tanks or more build liandrys, is the team very squishy build void staff, is the build very squishy and barely have any mr at all, build raba. If you built dark seal and its fairly stacked feel free to upgrade it.
Fifth item: if you havent already built void staff, build void staff. If you built void staff build raba. If you think you can abuse that they have a lot of ap damage dealers (3 or more) and dont feel like you need more damage and they dont have grievous wounds build spirit visage, if they have mortal wounds build banshees. If they are full ad , thornmail or an fitting armour item is not too bad.
Final item: if you added more damage last time i suggest you either go for the full glasscanon build and build the last damage item of liandrys, voidstaff or raba too nuke them before they have time to react. (I usually go full glasscanon because your rotation blow up swuishys before they can deal damage if you land your root and your healing scales with ap) or you round of with a deffensive item like banshees against cc, spirit visage against ap teams without mortal wounds or rylais if its a kitable enemy team and you have built liandrys. If last item was a tank item you build a damage item usually. Round off the build with a ellexir of sorcery.
Off meta things like seraphs embrace, mejais and glp works. Its just not that good or reliable in higher elos cause i rarely see pros build them. Mejais is to risky since you stand in the middle of a teamfight often with swain. Seraph is a risk because you are shooting up your core which works perfect for an item that is not realy necessary and therefore you powerspike harder but later for not a that huge reward. You need that midgame powerspike because that is when you are the strongest. Glp 800 works as well. I have tried it. But it is outscaled by 5 minutes for more ap health and mana which is realy usefull and the active wont help that much since the reason swain dont hit his w most of the time is because enemies are too far back or have a dash. Not because the q slow is not enough. Most of the time.
Runes: go for lots of ap or a few cd blues. But i would say ap in the runes outweight the perks of cd but that my opinion. I always have the spells when i need them and when i do have them i want one good shot rather than a possibility to use my abilities again soon. (Cd is good if you have to put on your ult more than once in a fight because it drains so much mana. But if you are smart about it you only need it on one occasion during the fight.
Masteries: use deathfire touch and the resolve tree. You may use cunning tree but i think its not as good.
Motivation why i dont suggest you to build spirit visage: spirit visage makes your heals stronger but is easily countered by grievous woundd. Then its usually better to build banshees that gives you ap which makes you heal more, deal more damage, gives you mr and a super strong passive. Almost always better than spirit visage for Swain although banshees cost 200 more gold