Hello an wellcome,
to my very first guide. First of all i should mentione that this is not a guide for beginners, and it wont explaine the rune choises and sumoner spell like others do. It is just hare just to show what items realy compliment swain. btw, i dont give sh*t what you think about it.

Items that complimemt swain:

(I will go alfabeticly from top to bottom and mention items that i think fits swain)

Abyssal scepter - realy good if playing vs alot of AP champions, gives a good amount of MR and +20 Magic penetracion (situacional item)

Archangel's Staff- It gives quite a lot of AP and Mana and Mana Regen. very usfull i would recomend to buy a second Aechangel's end game as a finishing item, becose your ap and mana whill rocket sky high. (must have item)

Athene's Unholy Grail- quit usfull, but i dont realy like it, Archangel's gives much more.

Deathfire grasp- must have item. combinend active efect whit your E deals a sh*t load of dmg. and should i say it gives -15% cooldowns? (MUST HAVE) dont forget to use it in teamfights

Lich bain- Realy good when you have a lot of ap, goes great whit Archangels staff.

Mejais soulstealer- very unstable item, can be very usfull, or if you die its a total waste of money.

Rabadons deathcap- should i say why it is a must have?

Rod of ages- an ok item, gives a good amount of HP, mana and AP, but it only becomes usfull if the game is over 45mins

Rylai's Crystal Scepter- Must have on swain. A good amount of AP, but what realy makes it good is the 35% slow on enemy's (MUST HAVE)

Will of the ancians- good if your team has a lot of AP's otherwise its not very efective.

Zhonya's Hourglass- good item when going against alot of AD's

Skill order:

Allways lead in whit E it gives you 8-20% damige increse on that specific target. follow up whit Deathfire grasp, R -> Q -> W

Hope i helped someone. sorry for my bad english.