Before you read this guide, I am not terribly good at League as a whole, but I love to play it anyways, a great, in-depth swain guide can be found here: https://www.mobafire.com/league-of-legends/build/fear-the-power-you-do-not-see-in-depth-swain-guide-523497

Hello, now before you look at my profile and see my rank, just know this guide is OP either way. Now, into it... I'm Steven4547466 (yes it's a lot of numbers) look me up if you will, I'm no one special. I've been playing League for about 5 years now, wondering why I suck already? In 2015-2016 I barely played because I got into other things. But now I unlocked Swain before his rework, specifically for the rework. Since then I have mained my man Swain. I am Mastery 7 with my boy now.
I used to main Garen when I started playing, that's why I've got all the points in him, but now I play a bunch of champions.
HOWEVER just like you, I'm not perfect I have my good and bad games and all the videos I will show you are not premades, but I was laying duo in some of them. Make sure to suggest anything you want added or changed!

Runes on Swain can vary, it comes down to personal preference a lot. However, here are my runes that seem to work very well.

Arcane Comet, I use this to help secure kills and to help with early pressure, if you feel like it's wasted with the E or Q, Aery is always a nice secondary pick.

The Ultimate hat. This is your bread and butter, the cheery on top, that sweet sweet ultimate that just tilts people "It's too strong," "It's CD is waaaaay too low!" they say, well no duh they are against The Noxian Grand General, this just makes it even lower of a CD and will leave them begging for mercy, but none shall be spared.

Transcendence. The CDR gained from reaching level 10 is just amazing, it means you don't have to waste a slot to get more CDR, with this build, you will get 20% CDR which is enough to wreck an enemy team with your 800 (really it's 749 with no drakes) damage Q.

Gathering Storm. The AP you get for an average 30 min game increases by 48 AP with this rune and it is worth it. Your win rate will only continue to climb as you get more and more AP to do more and more damage.

Cheap Shot. This is the obvious one, you deal extra true damage to immobilized champions, your root, yes, does make them immobile and you will deal a little extra damage with your auto attack.

Eyeball Collection. With this, the more kills you get the more AP you will get, in this case 2 extra AP per kill and 1 extra if you kill a ward, this is entirely up to you however any of the hunters or even zombie ward is a good choice.

Passive: Ravenous Flock
Effect Radius: 1000
Cooldown, Static: 12/9/6 (1/6/11 lvl, respectively)

INNATE - RAVENOUS: Swain can periodically target a nearby immobilized enemy champion to rip out a Soul Fragment from them, dealing 20 - 105 (based on level) (+ 30% AP) magic damage and pulling them X-units towards him.

Enemy champion Damage rating takedowns will also leave a Soul Fragment behind.

INNATE - FLOCK: Swain's ravens harvest Soul Fragments for him, Heal power restoring 4% / 5.5% / 7% maximum health. Swain can store up to 5 Soul Fragments in reserve, which empower Demonflare.

Q: Death's Hand
Effect radius: 725
Cooldown: 7.5 / 6.25 / 5 / 3.75 / 2.5
COST: 65 / 70 / 75 / 80 / 85 Mana

MAGIC DAMAGE:
55 / 75 / 95 / 115 / 135 (+ 40% AP)

BONUS DAMAGE PER BOLT:
15 / 20 / 25 / 30 / 35%

ACTIVE: Swain unleashes five bolts of eldritch power in the target direction, dealing magic damage to the first enemies they hit. Enemies can intercept multiple bolts, taking bonus damage.

Bolts that kill their targets continue onward and restore 3% maximum mana. This can repeat indefinitely until the bolts reach their maximum distance.

W: Vision of Empire
TARGET RANGE: 3500
EFFECT RADIUS: 325
COST: 70 / 85 / 100 / 115 / 130 Mana
COOLDOWN: 22 / 21 / 20 / 19 / 18

MAGIC DAMAGE:
100 / 150 / 200 / 250 / 300 (+ 70% AP)

SLOW STRENGTH:
25 / 35 / 45 / 55 / 65%

REVEAL DURATION:
4 / 5 / 6 / 7 / 8

ACTIVE: Swain summons a demonic eye at the target area, revealing it for 2 seconds, which explodes after 1.5 seconds, dealing magic damage to all enemies hit, halved against non-champions, and slowing them for 2.5 seconds.

Enemy champions are revealed for an additional duration and have a Soul Fragment ripped from them.

E: Nevermove
TARGET RANGE: 850
EFFECT RADIUS: 100
COST: 60 / 65 / 70 / 75 / 80 Mana
COOLDOWN: 13 / 12 / 11 / 10 / 9

MAGIC DAMAGE:
35 / 70 / 105 / 140 / 175 (+ 25% AP)

DETONATION DAMAGE:
35 / 45 / 55 / 65 / 75 (+ 25% AP)

ACTIVE: Swain launches a demonic wave in the target direction, dealing magic damage to enemies it passes through.

Upon reaching its maximum distance, the wave returns to Swain, detonating at the first enemy hit, dealing magic damage to all nearby enemies and rooting them for 1.5 seconds.

R: Demonic Ascension, Demonflare
EFFECT RADIUS: 650
COOLDOWN: 120
CAST REQUIREMENT: 1 Soul fragment

BONUS HEALTH:
150 / 275 / 400

DAMAGE PER SECOND:
35 / 50 / 65 (+ 14.167% AP)

HEAL PER SECOND:
20 / 35 / 50 (+ 16% AP)

ACTIVE: Swain frees the demon within for 12 seconds, gaining bonus health.

While Demonic Ascension is active, Swain drains the 3 enemies closest to him, prioritizing champions, dealing magic damage to them each second, and healing himself for each enemy hit, reduced by 80% against minions and monsters.


After restoring 125 / 300 / 450 health with Demonic Ascension, Swain can end its effects early to cast Demonflare, and automatically does so at the end of its duration.

If Swain dies while Demonic Ascension is active, he loses all of his stored Soul Fragments. If Swain dies during the channel time of Demonic Ascension, the cooldown and all Soul Fragments are fully refunded.

Demonflare:
DAMAGE PER FRAGMENT:
45 / 60 / 75 (+ 27% AP)

ACTIVE: Swain releases his stored Soul Fragments, dealing magic damage to all nearby enemies based on how many he accumulated.

Generally, I love to start with my E mainly because people don't seem to expect it and it allows you to get really good early aggression and even possibly a first blood. However once they know you started it they will probably play safer, so land that first one. If you don't land it, that's okay, just be around for the next 7 minion deaths and get your Q then go all aggressive on them. Although you start E, it isn't the first thing you'd want to max (in normal situations), you usually want to max Q for extreme burst damage, I like to max this at level 9 and by that time it's most likely doing 500+ damage. After that, you obviously want to get your E in there, it actually does quite a bit of damage, around the 250 range when maxed. It is useful because of that sweet CC and you use it to combo with your W. Your W , the back killer (literally). This will get you a ton of kills of champions that love to back with their shop open, never do that, if you place it on them when they are low and backing, even if they don't have the shop open, a centered W will probably hit. In addition, you can use this to stop people that are backing to continue to apply pressure.

Here's the fun part... Items! You start with the normal Doran's ring 2 health pots and a trinket warding trinket. On your first back, you should have more than enough for your Sapphire Crystal and if you do, skip the boots and move to the Ruby Crystal and if you have enough (1100) go straight for the Catalyst of Aeons then get your boots . Then, you want to move to the Rod of Ages , overtime this will be one of the best items in your kit. After that, you want your precious Zhonya's use this with your ult and wreak havoc. Next, you want to get your Liandry's , this is where all your damage over time will come from, the burn can really hurt an enemy champion. The extra damage while in combat is no hurt either! The Void Staff is the next item on the list the 70 AP you get from this along with the 40% magic pen will burn right through the enemies magic resist and guarantee you kills. If you get this far, congrats, but next item you want is Rylai's Scepter you mainly get this for the AP and slow, but health can always help in a battle. If you are having a hard time, try one of the situational items of course any item can be applied, but these can help. If you are losing because of a high magic damage champion, such as Lux, try out a Spirit Visage this will add that extra 55 MR, 450 HP and of course the HP regen. Having trouble with CC? Use a Banshees's Veil do note, you must not take damage from enemy champions for 40 seconds to get the spell barrier. If you just want more AP, switch out one of your items for a Morellonomicon along with the AP, you also get HP, magic pen, and grievous wounds on spell casts that hit enemy champions. Feel free to play around with all items thought, see if you can make the build even better.

I don't ever use the replay system, don't be mad at the way I used it
However, here are some of my victories on Swain, download them if you want and go over it yourself, a nice video on how to open old replays (.rofl) files can be found here:
Play Video
If you are reading this on the current (made: 5/18/2018) patch, all you need to do is drag the replay (.rofl) over the shortcut you made refer to the video to find out how to make the shortcut

https://cdn.discordapp.com/attachments/353624016642113546/447115019906187284/NA1-2785244847.rofl (Yes it's discord, it's free, easy and fast and it's a direct download)
I'm not sure how I played these games I just got 2 extra of my wins on swain from my 20 recent, but if there are dumb mistakes I make, just know, I'm B1, just do it better.
https://cdn.discordapp.com/attachments/353624016642113546/447118284035194890/NA1-2784709963.rofl
https://cdn.discordapp.com/attachments/353624016642113546/447118295175528448/NA1-2785169690.rofl
https://nofile.io/f/Vy1bFgxska0/NA1-2785863750.rofl
https://nofile.io/f/wwwcPJJM35b/NA1-2786420406.rofl

This would be an example of a good start, with help, remember this is a team game!
Play Video
Always be a proactive player, pay attention to the map and spot out where you can help your team survive and get objectives
Play Video
Most games have an assassin, this is how you counter one, try not to let yourself get this low, this Katarina didn't seem to know what she was doing, so I wanted to see how far I could take it. Play Video
Again, being proactive, this game I was fed, I went 14/0/1 when they surrendered. But this is a prime example of what you want to do when the enemy slips up. Play Video
I like to call this definitely not kill stealing, I was just "securing the kill" Play Video
I wasn't joking when I said arcane comet will get you kills, here's one. Play Video

Here I will talk you through everything I do when I get in game.

Early game: When you start, get your E (if you start E) and then get into lane, go to the opposite jungle side that your jg is starting, and stay in the bush, look for invading enemies and don't try to 1v3, get help from your team before you go in. (Starting will vary from time to time.) Next, when the enemy laner is in lane, make sure they are close enough for your first E to hit, this is crucial to getting early aggression. Once you pull them once, be aggressive (be be aggressive) if they don't learn to back off, punish them, at 7 minion deaths you get your Q and it does quite a bit in the early game to an assassin. Once you have got the laner dead and have at least boots and the Catalyst, you should have 2-3 points in Q, 1 in W and 1 in E. It's time to wombo combo them, once they are close enough: E them, put your W where you are going to pull them, then Q, this is the max damage you can get early game and its a lot of it. Try to make sure your Q can actually hit and wont stop at the minion line.

Mid game: If you are ahead, good, if you are behind, for any reason, it's time to play catch-up. If you're ahead, continue to be proactive and look for opportunities to get objectives. Farming is vital (I'm no good at it) the more CS you have; the more gold you have; the more kills you get; the higher win rate you get and deserve. If you are behind in CS, get going, you don't have the time to play catch-up forever and you will lose lane if you don't get up there. If you've already lost a turret, defend your second turret and shove the lane, find opportunities to gank other lanes, preferably right after you killed the minions and got your wave slow pushing, force the enemy to defend and 4v5 them or they will lose a turret. If your team is behind, but you are, being the god you are, ahead, go help a lane, pick a lane, ping you're on your way, push in that lane and get that turret, your laner will have to react and have the lane be shoved, or lose another turret, if they are pushing however, free turret for you and maybe them. Once you get their turret, move to the other lane and help them, a lot of people love to push only mid, it may be the shortest lane, but it's also the easiest to gank/pinch and the most predictable. Getting other lanes turrets means you have all the pressure you need to win.

Late game: If you get this far and you are ahead, you should be at around level 16/17 and if you are, continue with your team, work as a team and group, if you're at their nexus and you keep going in 1 by 1, the enemy team will just ace you and end up winning, you don't want that, group up, play as a team and focus. However if you're behind, you most likely lost your lane and have yours completely shoved into the inhib, in this case, get a situational item, damage means nothing if you can be 1 hit. Try to keep your CS up, your Q should 1 hit caster minions and almost 1 hit warrior minions. Remember you get a lot more gold when you kill minions, but don't let farming be the death of your team, if you are 5/9/12 and you see an opportunity for a winnable team fight, ping your team and hope to god they respond, at this point your R and Zhonyas combo is extremely good to stay in fights, make sure they know this, ping your Zhonyas and R are up. Be prepared to use your summoner spells, if you have to flash because they are just out of reach because you Zhonyas'd it's more than likely more than 2 champions and you (your team) should be able to get the kills (across your team, or alone) and then you are on your way to making a comeback.

Team work is a big part of League, going in 1v5 will never turn out well, unless your a Diamond smurf, in that case, just stop playing if you need to be satisfied by killing new players. Make sure to use pings, but don't spam, this tilts your team and gets them frustrated, frustration is completely normal when someone is being toxic and it usually spreads and fast, resolve the issue and work as a team. If 1 person is frustrated, chances are they end up doing bad which can transfer to anyone on your team because they can feed, then you get frustrated, then you feed and little Billy gets frustrated and the Domino effect continues. Just be a team player, it pays off. And remember, if someone is being toxic in the chat, chances are you can turn their mood around, just be a nice person, anger might spread, but so does niceness, this is a type of boomerang effect.

TL;DR: E, W, Q: Wombo combo, Be a team player, be proactive, be nice to your team, start E, max Q, Max E, Max W, Zhonyas + R = Death to enemy, don't be predictable, predict their plays, have vision, use situational awareness.