my name is shane or IGN IIShaneII
and this is my guide to how i build my swain.
as we all know swain is typed as an AP Caster, basically nothing else then a mage using Ability power for his damage.
swain can be a perfect dominator for mid, as long as he is build well and knows what hes doing of course.
the first 2 pages are completed guides to how i build swain, the third page is just a list of runes that you could use if you do not like the rune set up i have given you on the first 2 pages.
now the 4th page, is a build to swain that i have been starting to use more and more often, specially since swain really is mana hungry, following this fourth guide will give a lot more lane sustain mana wise.
the rest in runes masteries etc, all is the same, just a little different start build.

lets get to the runes then shall we.

for Red's
I would advice to take magic penetration flats, main reason amp's up you're damage a little early game, its main effects will be taking place later on when you penetrate more of the enemies Magic resist against you with it.

next

For yellow's
I would advice the flat Armour, if the enemy jungler is paying any attention you should be ganked frequently, having a good amount of start Armour to counter that is never a bad thing, since this only counts for lane phase.

The Blue's
Take stacking Magic resistance, this is just because the early game damage is not that much of a threat, so no need to have that instant high amount of MR i rather have it slowly stacking to keep up with the enemies rising AP.

Last up the Purple's or Quint.
Take Flat AP, this in combination with you're magic penetration marks will benefit both you're early as well as you're late game as you have more AP, meaning more Damage.

i take the 21/0/9 mastery on swain over 21/9/0 why, swain will have a good amount of sustain once he gets to level 6 therefor i do not see the need to take the extra Armour and magic resist over increased manapool size and quicker mana regen,(both in the utility tree called expanded mind and meditation)
also, these utility points make you slightly faster using the swiftness mastery.
and give you increased buff duration with runic affinity,
So longer amount of time that you can carry blue buff and dominate.
the 21 points in offense pretty much speak for them self, this is all for pure damage output.

you can level skills up to own preference of course, but the main reason i level my Q so my Decrepify
And E Torment
Together is for utility and damage purposes.

I consider W or snare(Nevermore)
To be a utility and therefore the lowest priority to level, and Q is you're slow and main source of damage next to you're ultimate or Ravenous Flock
This is highly recommended to level, as is you're E since this will amp up any type of damage output you have, there is my main reason to level then combined for the most balance between utility and damage.

Boots and pots pretty standard and almost every carry, this is because carries are usually slow, so the boots help evading enemy skills as well as engaging and escaping during lane phase. the pots are just there for sustain or to prevent yourself from dying.
now the reason i build up straight catalyst is because i want a quick Rod of Ages, this give me a lot of Health/ mana (which swain eats through once he gets to level 6) and some AP for damage.
the reason i rush a hextec revolver after this is for spell vamp, swains ultimate will heal him for a portion of the damage dealt, increasing you're spell vamp will also increase the amount you're ultimate will heal you for.
after that i get my tier 2 boots, more mobility as well as amping up my Damage output.
the next item is tricky, i usually go rabbadons first for a massive increase of my damage potential, but if you find yourself short on health or having troubles with enemies constantly running away from you build the rylai's first for utility purposes, it gives health a slow and still some AP.
after this get a voidstaf Arch Angels staff or Abyssal scepter to what you need.
still to low on mana get Arch angels, not dealing enough burst ? get Void staff having troubles with taking to much magic damage, take the Abyssal scepter, build that last item according to what you need.
after this finish will of the Ancients and buy the Elixirs and you will Dominate !

basically it is like this.
start the game with:

you're early game goals should be:

as this is a pretty good amount of gold to spend it will take a few minutes to farm up.
after this you will be looking at possibly 15 minutes in should be no more.
The Hextec revolver how ever is a true must have, this will increase all the healing done by damage from you're Ravenous Flock (ultimate)

this skill already heals you for a potion of the damage dealt by it, but with the hextec revolver you amp up this healing effect giving you a tremendous increase of lane sustain.

you should be turning towards building the following going on to mid game:

the arch angels is only if you can afford it, otherwise just get the tear, and start stacking it.

Towards end game you are going to build:

now that you're build will be complete you will want to buy the Elixirs
To just increase you're stats up a bit.

And buy an oracles and reveal wards with it.

these speak for themselves, flash.
can just get you in close enough to secure the kill, or flash you out of harmsway during a gank all the way to safety.
as ignite
can be used to counter a mid lanes spell vamp take morganna ignite wil really cut her regen of, as wel as secure kills due to it's true damage over time.