@Riot: Just drop the Infinitely scaling Rune Idea all together. You CANT balance it
Its fine(ish) in a champions kit, but as a Rune that anyone can pick up has ALWAYS been a bad idea.
This is what Dark Harvest should be.
Damaging a Champion, Large Jungle Mob, Cannon Minion or Epic Monster, below 50% health deals adaptive damage and harvests their soul, healing you for 5% of the targets Max HP (Max 400 on Jungle and Epic Mobs). Dark Harvest damage: 50-180 (based on level) (+5% of the targets MAX HP) (+25% bonus AD)(+15% AP) Cooldown: 40s (reset on takedown)"