Dark Harvest Is Impossible to Balance in its Current State

It will either be straight up game breaking or borderline useless, and it isn't because it has infinite scaling.

It's because it is incredibly easy to proc, and the condition for it to proc makes it always favorable. Added burst to low HP targets with 0 requirements is bad design, and goes entirely against Riot's keystone ideology of "runes shouldn't just be free damage".

Every other Keystone either has activation requirements or actual drawbacks, and isn't just free damage.

For comparison:

Precision

• Fleet Footwork: deals no damage.
• Press the Attack: requires 3 consecutive auto attacks to activate.
• Lethal Tempo: attack speed buff with activation delay.
• Conquerer: requires 4 seconds of being in combat and an auto attack against an enemy champion to activate.

Domination

• Electrocute: requires 3 independent damage sources, moderate cooldown after activation.
• Predator: requires a channel before activation and has a lengthy cooldown.
• Shitty Lethal Tempo Hail of Blades: attack speed buff on first 3 attacks against enemy champions, must go out of combat for it to go off cooldown.

Sorcery

• Arcane Comet: procs off any source of damage, but it can miss.
• Summon Aery: procs off any damage source, but deals little damage, and offers utility/support additionally.
• Phase Rush: deals no damage.

Resolve

• Grasp of the Undying: similar requirements to Conquerer without a theoretically indefinite duration.
• Aftershock: activates upon hard CCing an enemy champion, moderate cooldown and relatively low damage.
• Guardian: deals no damage.

Inspiration

• No damage keystones.

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Dark Harvest: Deal considerable damage to a champion below 50% health, activates on any source of damage, resets on takedown. This is definition free damage with next to no interaction, paired with infinite scaling / is a snowballing rune, this keystone will never be balanced.

It can't really be wasted, since the activation requirement makes it an execute / is always a favorable condition for activation.

While old Dark Harvest was no bastion of balance, it was infinitely healthier than it is currently because it actually had conditional uptime / activation.

Riot, if you want anyone to have any sort of respect for your supposed ideology and balancing, admit your mistake and revert Dark Harvest.