User blog comment:Ozuar/Swain's Passive/@comment-3974211-20130407141626

How about any of the following:
 * Whenever Swain kills an enemy unit with one of his abilities, the mana cost is refunded. For Ravenous Flock, this amount is the current upkeep cost.
 * The above, but in addition: Swain will refund 9 + (1 × level) mana if he kills an enemy unit with a basic attack.
 * Whenever Swain kills an enemy unit, he restores 4% of his maximum mana.
 * Whenever Swain kills an enemy unit, they will drop carrion. Beatrice will pick up nearby carrion (425 range) to restore 4% of Swain's maximum mana. (Swain can also pick it up within 150 range)