Before I begin, I would like to clarify that this is not a post crying about how Swain is underpowered, or that he needs buffs, or anything else of the sort. I love Swain, and think that, if anything, he is too powerful because of how high is ratios are. With that out of the way, let's get started.
What I think Carrion Renewal is:
A way for him to maintain a reasonably high amount of mana in lane to deal with the high early game mana cost of his abilities. This, in my opinion, is not where Carrion Renewal fails. I believe that it makes Swain's early game manageable, and simultaneously makes it so that you need to balance the amount of farming you do and the amount of damage you can deal to your lane partner. This adds a great depth of play to Swain.
Carrion Renewal scales poorly:
Although Carrion Renewal makes Swain have an interesting early game experience, it does not scale well at all into late game. To put this in perspective, the passive on Karthus's level two Defile will give him the same amount of mana for killing units that Carrion Renewal will give at level 18. Not to mention that Karthus has a much easier time of farming with minimal mana than Swain does. Also, a will give half of the effect that Carrion Renewal does at level one.
Perhaps change Carrion Renewal into something that provides an additional level of gameplay, such as Illumination does for Lux or Essence Theft does for Ahri. This would need to be something that gave Swain mana, considering that is what he really needs out of his passive (either that or something that made it so shields didn't so easily save people from Torment).
Draft of passive, suggested by the community, specifically Archonell and Emptylord:
This would allow Carrion Renewal to scale into late game, because it's a percentage.
Questions? Comments? I want to hear all of them below!