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 Champion 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 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 Crest of the Ancient Golem 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, MR 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.
For this reason, do not overlook getting a Deathfire Grasp in mid/late-game. Swain has tankiness, damage-over-time, crowd control, and self-healing. While Swain is very strong, the majority of his damage occurs over three or four seconds, which is usually plenty of time for the enemy to take action.Deathfire Grasp grants him the ability to crush single targets in combination with Torment.
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 extremely 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.
If you aren't satisfied with the one-hit-wonder characteristic of the DFG, a great alternative is Morellonomicon, as it provides greater amounts of the same stats without the restrictive active effect.
As almost all of Champion skills rely on positioning, Rylai's Crystal Scepter can be invaluable to keep foes in range of Decrepify and Ravenous Flock, and ensure landing Nevermove. As all of his spells are either DoT or AoE, however, it will only apply the 15% slow, making the item less efficient. But it gives a slow to Torment in the front of the spell combo when bursting enemy carries.
With the recent buff to its aura, Abyssal Scepter becomes an extremely viable item on Champion 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 120AP 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.
Liandry's Torment can be effective on Swain as it gives magic penetration, moderate AP, minor health increases and his moderate CC/AOE from his kit (especially when paired with Rylai's Crystal Scepter) have decent synergy with its passive.
Frozen Heart is an effective option. Besides offering a lot of armor, this item has mana (allowing Swain to mantain 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.
Of course, Frozen Heart is not an substitute, but an addition alongside Zhonya's Hourglass if Swain is in need of more armor.
Swain mostly deals magic damage, investing in magic resistance will mitigate his damage.
Do not underestimate Champion 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.
High movement speed and good mobility counters most of Champion spells.
When Decrepify is cast, quickly break the tether by moving away from the bird.
Avoid getting hit by NevermoveSwain 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.