If you're having trouble rooting an enemy with Nevermove, try using Decrepify to slow them down first.
While laning, try to gauge the strength of your opponents to determine the right balance of aggression with Ravenous Flock. If they're stronger, it might be more beneficial to stay back and cast Nevermove or Torment.
Swain can easily disable a single attacker. Coordinate with your allies when ganking him.
High mobility counters all of Swain's basic abilities: moving away from the Raven will break the slow, it is easier to dodge Nevermove, and you're able to escape focus fire while Tormented.
Due to Carrion Renewal offering mana, and Ravenous Flock offering health, Swain can potentially stay in a lane for a very long time. Note that Carrion Renewal cannot hope to offset the massive mana cost of Ravenous Flock, and his ultimate must be used sparingly. Combined with aggressive lane control, it is recommended Swain takes a solo lane to amass gold and experience.
Around level 4, you will have gained Swain's full combo. Torment and Decrepify followed by Nevermove and some basic attacks will devastate unarmored enemies. If you are too far for his other two abilities, using Nevermove first can allow you to close the distance.
Swain's basic abilities each have different benefits when leveled and maxed over the others.
Decrepify grants moderate damage and has a good slowing effect, on a short cooldown and good range. Ensuring foes stay trapped in the tether with Nevermove or allied disables is key. Ranking it will also make it easier to land Nevermove. The downside is that you are either forced to choose between safely using it at max range, thus making it easier to break the tether, or going in closer to more reliably get the full effect. It is also not as useful when your jungler initiates a gank, because the enemy has a better chance of breaking the leash.
As your main AoE and crowd control ability in one, leveling Nevermove makes it more usable by reducing its cooldown significantly. Alongside the damage increase, Swain's farming/pushing becomes much faster and easier, and the snare is absolutely amazing for countering ganks and assisting during one of your own. However, it is unreliable against enemy champions if one is not able to land it consistently, and if they predict you leveling this first, they can grab early Boots of Speed to counter the ability directly. It is recommended to begin ranking this skill if your laning is going absolutely horribly, and you do not believe you will be able to kill your enemy during laning phase. If you do not rank it and you are behind, the enemy will be able to out-push you and you will lose minions to your turret.
By itself, Torment offers nice harassment at a low mana cost. Even at low levels, you can take off 2-3 bars of health at a time using Torment followed by autoattacks against a squishier champion. In combination with Ignite, Swain can provide fatal damage output early on. However the damage dealt by Torment alone is slightly low, while your unleveled abilities will not benefit very much from the damage-amp effect. The best scenario in which you would level this ability first is against a squishy target that is more susceptible to the damage from it and Ignite. The worst case is against targets with high sustain or health.
While laning, try to gauge the strength of your opponents to determine the right balance of aggression with Ravenous Flock. If they're stronger, it might be more beneficial to stay back and farm and counter-harass.
While Swain has extreme killing potential, his main counters lie in targets who build large amounts of defense as well as having built-in sustain, such as Cho'Gath or Nunu. Swain's exorbitant mana costs mean that if he cannot kill his target, he will be wasting his time even trying.
For utility purposes, Decrepify followed by Nevermove makes for a nearly guaranteed root. The time taken to cast Torment can cost you your disables, so choose based on the situation.
Ravenous Flock is not channeled, and cannot be stopped by stuns, silences, or any form of crowd control. In fact, it persists during the Zhonya's Hourglass active effect (it still continues to drain mana, and can automatically turn off during the active). You can use it during teamfights to drain the enemies' health, as well as restore health on minions after the fight. With extra spell vamp and a Spirit Visage, one can nearly be an unstoppable force.
You should still be wary of disables preventing you from stopping the spell, especially if left on for longer durations; those few seconds more can quickly deplete your mana pool.
Be cautious when using Ravenous Flock to farm, as it opens you up to ganks, as you will have no survivability beyond trying to keep one enemy away with Q > W.
While escaping, you can activate Ravenous Flock, as it will heal you as your enemies pursue you. This can be very effective because the projectiles prioritizes champions and heals yourself 75-88% of the damage dealt to them. Healing with your ultimate while throwing down your slow and snare gives Swain an easy escape.
The best combo for Swain is Torment, Ravenous Flock, Decrepify, Nevermove. This combo is especially effective against melee champions near creeps, because you gain health from the creeps while you engage the enemy champion. This drains your mana more quickly than almost any champion, however, so make sure to buy some mana items to fuel your combo.
If Swain is doing well, giving him the Crest of Insight buff can greatly increase his effectiveness, and even if he is doing poorly, the buff could allow him to make a comeback or at least get back on level ground with his team and the opposition.
With all the control and healing you have as Swain, magic resistance is inherently more important and useful. This is because almost all physical damage is done over time, which your healing counters, and most of the significant physical damage is put off by squishy carries, who are almost free food to your combo.
If you activated Ravenous Flock when engaging an enemy remember to stop the spell if the enemy champion escapes out of your range, if not you could not have enough mana left to use your spells when you reach him resulting in a counterattack, being ganked, or allowing the enemy to escape.
A good harass method is launching Torment immediately after casting Nevermove, then approach and launch Decrepify and some autoattacks before retreating, this combo should deal around 40-60% of a squishy enemy champ's health leaving him vulnerable to a final combo of your abilities.
Rod of Ages is practically a must have on Swain, granting him a generous helping of his two most important stats: Ability Power and Mana. Its passive can enable him to stay in lane for as long as he requires, and when leveling-up during close fights the extra mana may be enough to squeeze out a few more seconds in Ravenous Flock. In addition, its Health stat works with any armor and magic resistance Swain might buy, improving his resistance to burst damage. Given the time it takes to reach full effectiveness after being purchased, you should aim to rush this item in most of your games.
Combining Rod of Ages with Athene's Unholy Grail will make sure that Swain can quickly recover the extremely high costs from his ultimate without having to return often to base, as even with Rod of Ages, his mana costs are extremely high. This allows Swain to spam his ultimate more often. While usually combining both is redundant on almost every other champion, on Swain it is not, due to the above reason. Remember that the more mana your champion has, the more effective it will be Athene's mana regeneration passive. But keep in mind that going for an early Rod of Ages + Athene's Unholy Grail route sacrifices a lot of Swain's damage potential. So it's recommended to save purchasing it later in the game when Swain needs to stay with sufficient mana but can't afford to return to base due to teamfights and to protect towers.
Throwing Torment and Ignite on a dying-yet-retreating enemy will almost always ensure their death, even if they are healed by a moderate amount.
It is also effective to throw this combo on a squishy carry like Caitlyn at the start of a team fight. The damage from this and Swain's other abilities is usually enough to put them into a retreat, cutting away their dps, and may even kill them.
With Morellonomicon's handful of useful stats, it will also negate any last-resort healing when near the brink of death.
A Swain build that is centered more around team-fighting and staying alive is Rod of Ages, Spirit Visage. With a build initialized by these items, building AP directly translates into more healing, which in turn allows for more damage. After these items, it would be wise to build Rabadon's Deathcap, and Zhonya's Hourglass to provide massive AP and the invulnerability of Zhonya's Hourglass. This build is perfect for protecting your carry and being an anti-carry yourself.
With the recent buff to its aura, Abyssal Scepter becomes an extremely viable item on Swain's once again, as all of his abilities save his W fall within the aura range. Take if you need some MR, or if the enemy's building some, though if they start building too much you may have to switch out for a Void Staff instead.
Zhonya's Hourglass is one of Swain's most core items: the AP is very much appreciated, the armor allows him to fulfill his role of wading into fights and draining health with Ravenous Flock, and most importantly of all, he has perhaps the best synergy with the active of any champion in the game, as his Ravenous Flock's life-drain continues to work through the stasis (assuming you have the mana for the upkeep cost), allowing him to pop it and just steal health for 2 seconds while invulnerable.
Frozen Heart is an effective option. Besides offering a lot of armor, this item has mana (allowing Swain to maintain his ultimate active for a longer time) and cooldown reduction. Because Swain usually stays on the middle of a teamfight, this item is used to its full potential, and the passive benefits your entire team too. This item will allow Swain to be effective against ADCs; the only drawback of this item is not offering any damage at all, unless when combined with Archangel's Staff.
Swain mostly deals magic damage, investing in magic resistance will mitigate his damage.
Use healing reduction effects such as Ignite or the passive from Morellonomicon to prevent him from healing a lot of damage from his Ravenous Flock. This strips Swain of his primary sustain ability and makes him much easier to kill.
Avoid going on a 1x1 battle against Swain as a melee or short-ranged champion, since Ravenous Flock lets him heal for most of the damage done. This allows Swain to win trades against almost any champion, unless they have a lot of sustain themselves.
Using a champion with higher range than Swain is a good idea. For example, an ADC with very high range such as Tristana or Caitlyn can kite Swain and avoid being hit by his spells, thus allowing them to kill him easily.
There are other 2 alternative methods of dealing with Swain. One is quickly bursting him down with an assassin such as Fizz and LeBlanc. Another is calling your teammates to focus and kill Swain. This is viable because while Swain is tankier than other mages, he is still not very tanky, and being bursted will prevent him from dealing enough damage and healing himself. Keep in mind that Swain can buy items such as Zhonya's Hourglass and Banshee's Veil, which will prevent him from being quickly assassinated. If you somehow fail to kill him, don't bother trying and just run away.
Do not underestimate Swain's damage; although he only has one burst skill in Nevermove, he can dish out massive damage over time if you allow him to. If you believe that you will be able to survive Swain's initial 4 seconds of damage, such as due to your own high lifesteal stat, (as well as out-damage his ultimate's healing) you may be able to tank his damage and turn the fight when he is waiting for cooldowns.
Swain is very weak early game against champions that can push the lane, due to his poor farming capabilities without Ravenous Flock. Being pushed to his tower will likely make Swain miss some lasthits, so pushing is a viable strategy, but be very cautious of enemy ganks, as Swain is one of the best gank followers.
Avoid getting hit by Swain'sNevermove to remove a large part of his damage combo.
Move away from Swain if he uses his Ravenous Flock. This will waste his mana and force him to put it on cooldown.