Welcome to my Swain guide.
My main point in this guide is to share my (in my opinion) broad experience with this champion.

1st build is constructed for experienced Swain players, and is only useful when everything is going well (minion farm, enough ganking either from you or others to help you stack mejai etc.) I think everyone understands what situation I'm talking about. With this build you will most likely carry the game to the victory.

2nd build is quite defensive, which is good for Swain beginners and for games when you do not do so good as you would've expected (having hard time minion farming, not getting early kills or assists and in the worst scenario, repeated dying). As this build strongly improves your survivability, it lessens the probability of dying in big long teamfights and even increases your flexibility and usability in big long teamfights.

These runes work the best for the most mage champions. Nothing more to say.

I find Ignite as a base, motherboard, or whatever you call it on Swain, you just pick it, that's it.

• Cleanse I used to pick this one alot, but now I can tell this is the spell you pick when playing normals, going more tanky (see 2nd item build), playing more just for fun.
• Flash I find this spell the second main base for Nuke type of playing (1st item build), and for ranked play, in ranked Flash > Cleanse (and basically anything else).
• Clarity I've seen really alot Swain players pick it, but same as the 2nd item build, its more of a thing for Swain beginners. The thing is, if you stay in Raven form for long enough to lose most of your mana, clarity won't help, if you keep the form, because the mana cost is so high, you'll most likely lose the mana gained by the spell in the next 2-3 seconds.

I hope that the reason I made 2 item builds is clear from the Introduction section. But here I'll list optional items, which i find more or less useful, but the usability of these items basically depends on every player's playstyle.

• Archangel's Staff I know many would say it's a must-buy, which is not really truth imo. It provides a large amount of mana pool and mana ragen, (those two I find on this item the most important on this item) but if you play Swain for long enough, you will come to the point, when you don't ever need this much mana. And there's one more reason, Tear of Godess isn't quite easily stackable on Swain, most games last for ~30-35 mins and by that time you still won't have full stack of mana, which is quite a loss isn't it? Yes, in this item build I build absolutely NOTHING what gives you mana, but face it: if you can't control Swain's mana pool well enough, there's the second build waiting for you.
• Lich Bane This is a great item, and I mean it. When i play against team with strong nuke casters, I prefer to build this item prior to for example Spirit Visage, Banshee's Veil (which I don't find any useful). This also gives you an opportunity to change your playstyle.
• Will of the Ancients You may like this item, I build it alot when playing more defensively (via 2nd build), but still I decided not to put it on basic build list. It provides you really great survivability, combined with other defensive items and Raven form you completely change the definition of "tanky caster".
• Spirit Visage Useful item, only via second build (defensive one). For the first build it'd be a waste.