I have been enjoying Swain after his rework. Initially when I was looking at Swain's Q scaling and CD I thought building full AP damage was the only choice. By full AP, I meant a build order resembles ROA, Zhonya, Rylai, Morellonomicon, Banshee. However, as I tried more and more of these, it frustrated me to see Swain keep being useless in late game. Unlike other mages, Swain's damage output is every slow. He has to run into his enemies in order to unleash the full potential of his ult and Q. Full AP Swain gets destroyed so easily in teamfights (especially in a team in which Swain is the primary frontline). While Zhonya might help you to survive for another 2 seconds, depending on whether you have the chance to activate Zhonya while being CCed repetitively, you will be dead no matter what after that even if you get some of your HP back from your ult.
Then I have experimented semi-tank builds with better success. By purchasing ROA and Zhonya first, you still get to slaughter your enemies during the mid-early stage of the game. By procuring tank items for the rest of the game (Spirit Visage, and other Armor+Health items), you can make sure you survive long enough in teamfights even when your power start to falloff in late game. After building half-tank I realized that my damage after every match increased instead of decreased and I was able to achieve a higher crowd control score because I survived longer to spam more QWEs and passives.
It was at this time it dawned on me that Swain had a very high built-in damage in his kit. We know that Swain and Fiddlestick are the only two mages who have more than 1 hard CC among their non-ult abilities. If fiddlestick works best as a support, why can't Swain.

In one of those boring games in which every one of your lane slaughters their opponents you may not do much damage. But in even matchups, I am pretty sure there will be some non-support teammates who does a lot less damage than you.

The profile stats panel come out about a week ago. By comparing my tank support Swain's stats (16 matches) with NA server's platinum support Swain. My KDA is higher, damage per gold is a little higher, damage share is a little higher, utility score is significantly higher. It can be possible that on average I just had better teammates and better games. But conservatively I can say that you may do a little less damage playing support tank Swain than support AP Swain, but you contribute way more to your team by building tank.

A selection of some runes:

Arcane Comet:
Based on my average end game stats, Arcane Comet usually makes up around 7.5% of the total magic damage my Swain can do per match. Aerie is usually at 6% (pre-8.15 nerf). For the same reason you don't want to take Electrocate anymore because it is just nerfed, too.

Glacial Augment
Very reliable slow, helping you to land your E. However, it does not provide any damage. If you and your adc outdamage your opponents at laning phase this is a very good choice.

Unsealed spellbook
Swain can make use a lot of summoner spells other than ignite and flash. Ghost, exhaust, heal are all good choices. But again this rune doesn't help you much during laning phase.

Kleptomancy gives about 600 gold value at 30 mins it is next to useless. Before 15 mins the chance is about 25% to obtain a loot. Post 15 mins it is about 35%.

Phase Rush has a CD that is too long, a duration that is too short, and an effect that is mediocre.





Normally I will go
Arcane Comet
Mana Band - You don't build much mana in your core items so use this
Transcendence - You have 60% nominal CDR with full support build.(assuming no CDR boots) This is equivalent to 40 AP
Gathering Storm - Another free 48 AP at 30 mins mark (again not helping your laning phase but since scorch is nerfed ......)
Magical Footwear - Free 300 gold at 10 mins mark
Biscuit Delivery - Another 160 free mana and it helps to ease your laning phase a lot

You will have 20+40+48= 108 AP without any AP items for this build. (Can be more at 40mins, 50 mins .......)

or you can do this if you are obsessed with Resolve bone plating

Arcane Comet
Mana Band
Transcendence
Gathering Storm
Chrysalis
Bone Plating

You will have 10+40+48+15= 113 AP without any AP items for this build.

Taking Sorcery+Domination grants you the most free AP 20+30+40+48=138
But I don't see a reason doing that when cheap shot and electrocate are nerfed.

Zeke

Useful stats, armor, MR, mana and CDR. Cheap price. Extremely OP on Swain support if your adc is someone like Ashe.
After you cast your ult, it provides an 20% AOE slow around you and it grants your adc 50% more AA damage (in the form of magic damage, not including on-hits). If your adc hits the target slowed by you the AOE slow becomes 40% and you deal additional 40 damage/sec to the target (who are slowed by you and attacked by your adc).
The 3 reasons you don't want to rush this item first:
1) The bot lane partner is not AA based (Heimerdinger, AP Kaisa, AP Varus)
2) You and your adc get annihilated by an enemy ad bot lane (buy Knight's Vow instead)
3) You lose all faith in your adc. There is no way he can carry your team this match.

Knight's Vow
Useful stats, armor, health,and CDR. You don't buy it because the damage reduction to your adc is so critical. You buy knight's vow because its dirt cheap price and all-around stats.

Spirit Visage
Big chunk of health and some MR, significantly increase your ability to soak up damage.
After finishing Spirit Visage and the two items above you may have already become the tankiest person on your team, just not as tanky as a professional mid game tanks like Sion, Maokai or full tank Urgot.

Warmong
You have armor from Zeke and KV, MR from Zeke and Visage, you just need health to help you survive the teamfights.

Shurelya
Cheap and good stats. You build this only if your team is in lead and you want to help secure your lead by helping your teammates reaching the enemies. In other games you may not need this.

Remember these items are expensive for you as a support so you want to buy as less of them as possible.

Zhonya
Normally opponents do not focus a support item Swain because they see you as no threat. If you have to tank the whole enemy team just die for your team. You can't help your adc when Zhonyaing in the middle of a fight. Nevertheless, Zhonya is the most useful AP item for support Swain other than Shurelya.

ROA Rylai Morellonomicon
You are getting so ahead at laning phase and early game that you have abundant gold to spend. Then yes, you can build some AP.

Abyssal Mask
Somehow the enemy team makes you think the MR from Zeke, Visage is not enough.

Randuin, Deadman, Sunfire, Thornmail
You build one of these if you do not build Zeke in order to compensate for the armor loss. Depending on the matchup, all 4 of them can be optimal. Thornmail is a counter to high attack speed ADs. Deadman increases your walking speed. Sunfire is most effective when the enemies have a lot of close-quarter champions. Randuin has an AOE slow to help your adc.

Mikael
Great counterto Zoe pre-8.16 but Zoe just sucks now.

Sorcerer's boots offer damage. CDR boots offer 20 ability power with transcendence, 200 gold discount and 10% CRD to summoner spells. Mercury boots offer MR and some tenacity. These are all great choices

The worst choice to pick is Ninja Tabi. At 200 armor (67% physical damage reduction), the nominal 10% AA damage reduction decrease down to an effective damage reduction of 3%(10% reduced by 67%). You will get around or more than 200 Armor with Zeke and Knight's Vow.

High lane threats:

Zyra, Brand
These poke-heavy supports has CCs that are more reliable than yours. They outdamages you and they outranges you. Luckily, there aren't too many Brands and Zyras in ranked.

Pyke
While Blitz, Pyke, Thresh, 8.16 buffed Leona, Nautilus can all be threats to Swain, Pyke is the most fearsome among them. Pyke is resilient to Swain's poke damage - It can heal them up after being invisible and leaving combat. Supports like Thresh may get poked too hard at lvl 1 or lvl 2 that they are unable to intial any winnable fights later. Swain and his adc should be able to get to lvl 2 faster than Thresh and Thresh's adc.
There are a lot of Pykes in ranked, but you don't want to ban Pyke either because it is better to save the ban slot for OP junglers like Eve.

Blitz
You just don't want to face Blitz when you are a magnet player. Even when Blitz gets poked down to half HP he can still win fights if he successfully grabs your adc at lvl 2.

Lucian, Draven, Miss Fortune
Sometimes the enemy adc is just a lot stronger than your adc at laning phase. Especially when they are accompanied with someone like Leona, Alista.

Yasuo
Can't successfully land E and W.






Another strength of Swain support is that if Swain is picked firsthand the opponents can't realize Swain is the support. Swain can be any of the other 3 roles except for jungle. When seeing Swain the opponents can be tricked into thinking there is a Swain adc or mid, thus compromising their counterpick strategy. This works especially well if your team's mid picks the last.
If you are picking support after you see the enemy support, you should counterpick instead of lock-in Swain as if you play nothing but Swain. Swain is a good counter to weak warden supports like Tahm, Taric. Swain also has a slight advantage over enchanter supports like Soraka, Lulu.

Lvl 1
Always learn E first. If you land E perfectly both damage will be triggered. Then you pull the target towards you, combined with some AAs and ADC's damage it will take down about half of your enemy's health. Sometimes results in a kill with ignite. It is always easier to grab an opponent with E when they are trying to poke you or CS.


Lvl 2
Same as lvl 1 but you have one more source of damage from your Q. You are stronger than most supports at lvl 1,2,3, with both sustain and damage. You want to fight as much as possible. It is a lot easier to generate a kill at low level than lvl 4 and lvl 5.


Lvl 6
You just suddenly become a lot more tankier. In early teamfights you can be both a tank, damage dealer and has 2 hard CCs 1 AOE slow. If the enemy team has only one source of damage (physical/magic), your power spike can usually last until 25 mins-30 mins in an even match (normal scenario, nobody gets too fed).