A Point Worth Making About Swain
If you look at Swain's kit and what he does, he actually bears a striking resemblance to a juggernaut. Rather than specifically nerf a champ with enough obvious problems that he's barely played, why not embrace his strengths and round him out? For example:
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Replace his passive. Having a mana regen passive is far less an identity-defining ability than it is a band-aid over the obscene mana costs he was given. Even with that passive, Swain players sometimes feel they need to buy
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Buff his defenses while in his ultimate. Swain needs too much of too many stats. He should either be fine focusing on HP items or fine on resistance ones
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Shift his spell range. Make him get closer to Torment and Decrepify, but increase Decrepify's tether range if needed. Reward players for kiting and picking short trades effectively, punish them for greedily slug it out with a sustained DPS champ
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Emphasize the Master Tactician. I strongly recommend focusing his defensive buff while ulting on some in-game factor. Maybe make his self-buff slightly stronger when near an ally with low hp and increases as more allies get low. Now he's a champ who punishes tactical mistakes in not finishing off his weakened allies.
NEW PASSIVE RECOMMENDATION - Carrion Call: Swain's spells cause spectral ravens to pick away at enemy champions for (+/-) 2 seconds. The crows steal X% of all healing, lifesteal and regeneration to afflicted champs. Reduced effectiveness if multiple champs are debuffed.Heal reduction does not stack with Grievous Wounds
This passive change allows for Swain to be less reliant on ult for sustain (you could remove it). It also gives him an identity as the anti-sustain champ. Doing it this way makes Swain less of a snowball champ who thrives when he's already winning and turns him into one who's at his best when he's not. Is his lane a little weak, leaving him a bit behind? It's fine, maybe he's baiting you into his trap. Just like he did to 
