Simplifying the reason why dark harvest is too good.

It stacks with multiple dark harvests. Which is why bot lanes like Karthus/Xerath are absolutely disgusting, because both players can proc the passive. If you have a literal full team of dark harvests, you instantly have an advantage in a team fight if they don't.

Any amount of damage can proc it. ANYTHING. Passive damage (teemo poison, brand's flames, twitch poison, etc), item damage, (Liandries, Luden's, etc), ignite, smite, there are too many ways to cause the dark harvest stack to explode.

It resets on kills. Even if you don't get the kill, it resets on assists as well.

Because it's an infinitely scaling ability, it's value will never fall off, only getting stronger and stronger as the game goes on. If it only scaled on it's own damage, it would be a little bit less oppressive, but it also scales on your adaptive damage.

It essentially gives every champion execution damage, which is horrible because if they already have an execute like Veigar, Pantheon, Garen, Lee Sin, etc, it amplifies the kill pressure.