These runes provide a lot of AP, because this midlane Swain deals a lot of damage and tanks less than Swain toplane. You don't need to take cooldown reduction on Swain as one combo will take down nearly any champ and then you stay back till you get your cooldowns back. I'll explain it later in the guide. I prefer taking scaling magic resist than pure magic resist because most AP champs start dealing damage in mid-late-game, so taking scaling MR will give you a lot more magic resist/tankiness when you'll need it. For armor though, AD champs put more damage in early than in late ( Zed, Yasuo...) In Toplane, Swain needs way more tankiness and less AP than in midlane because he will fight against Off-tanks or even tanks. These runes provide more armor because you will fight against an AD-Offtank like Renekton. You still need a lot of AP to be useful in late-game or in case of gank. I mean it's good to be invincible and "unkillable" but if you deal no damage, you're not a real toplaner. Against an AP champ, you will need more magic resist. There, I don't take scaling magic resist unlike in midlane because the other ennemy will probably harass you and to last longer you need pure magic resist. In late-game, you'll probably face more AD champs and so you need some armor in your Runes. Keeping in mind you need AP as explained above.

As a midlaner, you must think and act like you are a midlaner. ( LeBlanc, Fizz,etc.) That way, you need a lot of AP and less tankiness in runes and masteries. Starting from the beginning. Prefer Feast to Fresh Blood as you will very rarely have the range to autoattack. (ennemies will be scared of you) You need both Vampirism and Natural Talent to get scaling AP (and AD but that isn't really important) and spell vamp that cumulates with your ult Ravenous Flock and gives more sustain. Then, I prefer Bounty Hunter to Double Edged Sword or Battle Trance because you will do more damage in late-game and this Swain Midlane is full mid-late-game and will not do damage only in 3 seconds but in more with ult. I prefer Precision to Intelligence because Swain doesn't need cooldown reduction because after a combo even with cooldown, you don't have time to be effective before 8seconds approximativly, so it's another fight. I prefer magic penetration and all-in.

Archangel's Staff: This is one of the key of the build. It gives scaling AP and Mana, that will give you a lot of damage and with that much mana, you will be able to activate R longer in teamfights. As this item needs to be stacked, you should build it as your first item. Rod of Ages: This is your second item. Giving AP, Mana and HP, you have everything you need to hold a lane and resist ganks. This item also needs stacking and will give extra-AP added to the Seraph's Embrace's passive. Both make you a really strong mid-late-game champ. Abyssal Mask: As a third item, this will really increase your damage dealt by Ravenous Flock, give you AP, a minimum of cooldown reduction, and magic resistance, often very useful as 80% of midlaners are AP. Build this as after your boots. Liandry's Torment: This item is really useful with Swain and will deal a lot of damage and give you extra-HP. The tankiness and long-term damage will burn everything arround you, making it easier to farm and kill in teamfights. Build this right after you build Abyssal Mask. Rylai's Crystal Scepter: Rylai's passive slows down everything arround you with your Ravenous Flock. Added to Liandry's Torment, the ennemies will be really weak arround you. Slowing down the ennemies will make it easier to catch them or escape. Zhonya's Hourglass: I don't recommend building this as the passive is not as worth as the other items i advice you to build. Indeed, your Ravenous Flock will still run when you zhonya's, but you will deal less damage or have less mana depending on what you replace. Boots: As a midlaner, I recommend using Boots of Swiftness more than any boots because you will probably be focused and you need to be able to catch up ennemies and escape easier. If the ennemy team has a lot of CC-champs ,prefer building Mercury's Treads of course.

Turbo Chemtank: You might be surpised by this item, but if you read carefully the passive, you'll see it's actually OP with Swain as you're nearly always using mana with Ravenous Flock. Moreover, the extra-movement speed provided by this item makes Swain much more mobile in case of ganks, or in teamfights. At the end, activate it near ennemies and get them slowed down. Sunfire Aegis: Aswell as giving you HP and armor to resist to your AD-toplaner, this item's passive deals additionnal damage arround you, which is really useful in teamfights but also to farm because the damage is increased to 150% against minions. Try staying close to what you're aiming with this item. Everfrost: I know a lot of you probably play Swain with this item but it has the same OP passive as Turbo Chemtank and I much prefer the extra-movement speed that can apply to all ennemies arround than a spray of icy bolts that won't help you escape and are hard to place correctly. Boots: Against an AD-Offtank, use the Ninja Tabi so that you don't feel as many damage and tank more when they attack you. In late-game it will also protect you from the adc if he tries to focus you.

Spirit Visage: Giving you magic resistance will able you to face the toplane AP-champ. It also gives HP, base health regen which means sustain, cooldown reduction making you really effective in late-game and the passive that will increase the Spell vamp of your Ravenous Flock by 25%. Boots: Against an AP-champ, you will need of course to take Mercury's Treads so that you don't feel as much their CC and slows (if they have some) and their damage.

• First, you need Flash in midlane because the abilities are much more "defined" and "precise" so if you want to dodge a spell like midlaners use ( Flash Frost, Command: Attack or Dark Sphere for example), you need a Flash when you're low-HP. In Toplane, you are much tankier, so harder to kill, and the abilities are less "defined" in Toplane ( Cull the Meek, Siphoning Strike or Crippling Strike) so you can escape by having movement speed easier in Toplane.
• Secondly, in Toplane, you will probably be more ganked as you are hard-pushing and harassing the ennemy. Ghost's cooldown is half of Flash's cd. That way, escaping every ganks can be much easier with Ghost. In Midlane though, the jungler will probably gank less but better, so flash as soon as he ganks and be safe.

The technic I use to farm is I place a Decrepify and a Nevermove for the 3 distance-minions behind and I finish the front-ones with my autoattacks. You will need Savagery to be able to farm correctly and not be too late compared to your opponents. Don't hesitate using your abilities when farming because you have a lot of mana and Carrion Renewal will give you back the mana spent. In late-game, use your ult to farm, but when you finished the wave, don't go to far because you will have Ravenous Flock in cooldown.

• EARLYGAME
  In laning-phase, harass the ennemy with your Decrepify to stop him from farming. Dont be too aggressive though, you are a late-game champ. If he engages a fight by coming closer to you, punish him using this combo:
  
  , put autoattacks when you placed every ability and he will go back and understand your damage.
  MIDGAME
  When you start getting fed and your items start getting stacked, you will be really strong and the ennemy will already fear you and probably won't 1v1. When you are ganked, use this combo: (focus the jungler to stop him from coming back after)
  
  and run back to safe-zone till jungler goes away.
  LATEGAME
  : In late-game and teamfights, you should always fight using your Ravenous Flock. If you don't have it, go back and wait. The combo should focus the champ who deals you the most damage: (Jungler or ADC)
  
  and stay arround the ennemies who have the lowest health.
  You might have noticed now you use Decrepify before Nevermove because the movement speed in late-game makes it really hard to correctly place your Nevermove and you really need it. Slowing down the ennemies with Decrepify will simplify it for you and won't take off any damage. The reason why I only put Torment at the end is that the inreased damage will be stronger and more efficient if the abilities dealing the damage are already placed.