What is my role when using this build? - While using this build you will have enough DPS to nuke a single target and enough survivability to off tank. Provided you master Nevermove, you will also have a good amount of utility.
Why use Swain when I have access to other casters like Morgana? - Although many seem to disagree, Swain's single target DPS is not lacking at all. If you do the math, you will see that while it may not be as high as the DPS provided by champion's like Veigar, it is still quite high. Because of your low ratios you have the option of building Swain as an off tank, giving him a large amount of survivability and a very nice amount of DPS when using Deathfire Grasp and Ignite. Furthermore, because his snare is not a skill shot and can hit multiple people, you have more flexibility than you would when using a champion like Morgana.
Would you say that Swain is underpowered? - At the moment, Swain is not as good as he could be. Given enough time and practice, I believe Swain could see competitive play, but he's definitely missing something that could make him a tier one champion.
Carrion Renewal - Swain's innate really shines during the laning phase, essentially giving you double the MP5 of anyone else on the map. Provided you're getting last hits every 6 seconds or so, which shouldn't be too difficult to manage. Combined with your ultimate, you can stay in a lane for gratuitous amounts of time.
Decrepify - More often than not you'll be initiating fights with this, but if you feel that you can catch up to the target without it, then use it after Torment to maximize its damage. You should always use Decrepify before casting Nevermove. With a maxed out Decrepify, if you position your Nevermove so that the target is directly in the center, it will be physically impossible to escape without a flash.
Nevermove - Out of all of Swain's spells, this one will take the longest to master. In order to use it effectively you'll have to memorize the time it takes to pop and learn how to control your opponent's movement. Due to the large range on Nevermove, using it to catch enemies running through bushes is very easy. Try to position yourself so that the enemy can only run to you or get caught in the snare. An enemy that's heavily concentrated on someone else might not notice Nevermove's animation, take advantage of distractions as much as possible.
Torment - Pretty basic spell. Try to use it first to maximize your damage. If you can't use it first, it isn't a big deal. At level 18, the standard spell rotation for maximizing damage output is as follows. R -> E -> DFG -> Q -> W -> Ignite.
Ravenous Flock - If you plan on tower diving, using this in conjunction with Flash will make your life much easier. With all three ravens hitting something, you'll be getting healed for almost as much as the tower will be hitting you for. If an enemy is using an Banshee's Veil, Ravenous Flock will allow you to easily break the shield and use your standard spell combo afterwards. Keep in mind that if you launch a raven at a target and they flash away, the raven will keep chasing them until it hits. It should also be noted that you will not be healed until the raven model returns to you. Using Ravenous Flock on creep waves is a good way to restore your HP while laning, just make sure you'll have enough mana left over afterwards to save yourself from ganks.
II. Skill Sequence
Not much to cover here that isn't already covered above, but if you're going up against a melee champ while laning then get Nevermove before Torment. If you're laning against a ranged champion, then get Torment before Nevermove. The standard skill sequence applies afterwards.
I imagine some of you may be confused by the fact that I don't max Torment as fast as I could. I do this because Torment deals less damage over time than Decrepify and Ravenous Flock. In order to maximize damage while laning, I always keep Decrepify one rank behind Torment so I can still max out Torment first, but make the most use out of its flat damage bonus.
Keep in mind that any item in this build, with the exception of Deathfire Grasp and your boots can be bought before or after the others based on the composition of the enemy team.
Right off the bat you're going to want to start out with a Doran's Shield and a Health Potion to increase your survivability since you'll be squishy for awhile. If you can complete one of your bigger items by selling your Doran's Shield, do so.
Immediately start rushing your Deathfire Grasp. Because most of your damage is dealt via DoT's, the burst damage from Deathfire Grasp is absolutely necessary to this build, especially due to the large lack of AP. You may want to consider buying your boots before completing this if one of your opponents buys their boots first. Make sure that you use your DFG while your Torment is still ticking to maximize its damage.
After your Deathfire Grasp is done, start working on your Mercury Treads. Even if the enemy team is mostly composed of AD champs, you will still want to get Mercury Treads to minimize the effect of your opponent's CC.
Depending on what the enemy team is composed of, you should acquire either of these after your boots. If the enemy team has a large amount of AP champs, you should buy a Spirit Visage. If the enemy team has a large amount of AD champs, you should buy a Randuin's Omen. Keep in mind that if the enemy team can reduce the amount of healing you receive, a Spirit Visage will be useless to you.
After getting either your Randuin's Omen or your Spirit Visage, get a Banshee's Veil or a Thornmail based on the composition of the enemy team. Same idea as above. If you're still having trouble against the enemy teams CC even after buying your Mercury Treads, buying a Banshee's Veil may be even more beneficial than a Thornmail against an AD heavy team.
After you've got either your Banshee's Veil or your Thornmail, get an Abyssal Scepter or a Guardian Angel based on the composition of the enemy team. Same idea as above. If the enemy team is stacking Magic Resistance, you should get the Abyssal Scepter in place of the Guardian Angel, even if the entire enemy team is composed of AD champions.
If you have enough time to acquire all five of the items listed above, then your 6th item is entirely up to you. Build whatever you think will be the most useful to you.
If you find that you're running out of mana very quickly even with your Golem buff, you can use a Chalice of Harmony as a fill in for one of the items with magic resistance in this build. You also have the option of filling your 6th item slot with this, but I wouldn't recommend it.
There are alot of items that synergize well with Swain, but I would suggest staying away from items with Spell Vamp on them. Although the amount healed for will be the same by the time the DoT ends, the small ticks of HP are much less potent than a sudden burst of HP would be.
- Early Game: Levels 1-10
Typically you'll want to be laning with a partner, but if your team doesn't have a good mid champ, you should be perfectly fine on mid if you don't do anything too anything too greedy. Focus on getting last hits early on to recharge your mana and grab some gold. Use Decrepify if an enemy gets too close for comfort. At level 4 you can start doing some serious DPS, but chances are you won't get a kill until level 6 if your enemies are good. If you end up laning against a champion with a built in flash like Ezreal or Kassadin, you should see if it's possible to switch with one of your teammates, as they will be able to easily escape Nevermove. If at all possible, you should try laning with a champion that can buff your magic resistance and armor, like Mordekaiser or Galio. The extra damage soaking is very useful due to your lack of defenses early in the game.
- Mid Game: Levels 11-15
Hopefully you've taken out a turret by the time you hit level 12. If not, keep pushing your lane until you do. Keep in mind that at this point you're a very effective ganker when using Flash. Switching lanes for a short time can be very effective provided you've managed to hold dominance over your lane. This is also around the time that the Golem buff becomes invaluable to you, allowing you to spam Ravenous Flock for extended periods of time when combined with Carrion Renewal.
- Late Game: Levels 16-18
At this point you should try your best to keep a Golem buff up on yourself at all times. The mana regeneration will be absolutely required when pushing the enemies base turrets. This is also around the time your damage begins to slowly diminish, making your mastery over Nevermove a key component to winning the game.
Not much to make note of here, but you have the option of using flat HP quints in place of magic penetration quints. All other runes should be kept as they are.
V. Summoner Spells
Ignite is required on Swain to maximize your damage output and to compliment your other DoT's. If someone is running away from you, chances are they won't make it very far.
Flash is an excellent choice for your secondary summoner spell. It makes diving towers, initiating, and escaping much easier on you when used in conjunction with your skills.
If you're having trouble when getting focused, using this in conjunction with Randuin's Omen can almost guarantee your escape. Making this a very good choice for your secondary summoner spell.
Most of the time I choose not to run this, but if none of your other teammates are using Clarity it's a good idea to grab this in order to refresh everyone's mana pool later in the game.
If you know how to use this skillfully, Clairvoyance is a very good choice for your secondary summoner spell. If you don't have experience with it, let somebody else on your team get it.
All other summoner spells pail in comparison to the ones listed above when playing Swain.
Golem Buff - When the laning phase ends, the Golem buff will become much more beneficial to you, but you shouldn't ignore your innate even though the Golem buff will provide more mana regeneration. Combining your innate with the Golem buff can keep your mana going for as long as you need, which will be absolutely required when pushing the last few base turrets. It might be a good idea to indicate that you'll be needing a Golem buff before you take it so the rest of your team doesn't try to take it before you. If there's somebody else on your team that needs the Golem buff, work together with them to take out both Golems on the map very quickly so you can both get the buff.
Lizard Buff - You should never have this on you unless you kill someone on the enemy team that has it. If you're ever at the point where you're forced to use melee attacks, you should instead be positioning yourself for your next spell combo.
Please ask any questions you have in the comments. I'll try my best to answer them. If you also think the build could be improved in any way, please let me know.