Dark Harvest is Riot's attempt at redeeming Feral Flare.

To all of my OG jungle friends who remember feral flare (rip patch 4.5-4.20 may you blaze it in glory) It encourages junglers like Yi, Udyr, Kayn etc who love to farm early and scale, in some regards, into the game. However the main issues with feral flare were A) it was so underwhelming early that you had to stack it for the first 20 min of the game and if you didn't you were basically screwed B) it was so good late game you could just afk farm for the first 20-30 min and not get punished for it even if your team lost all lanes and finally C) it turned any jungler who bought it into an item reliant jungler how the earlier iterations of rageblade/bloodrazor pre changes. However DH also solves the key issues that these items created with their playstyles. While it does encourage farming to stack it's on hit damage you only get the benefit under certain conditions which only become relevant with lane presence and farming combined. I feel that this is actually a really good idea for a stacking keystone that can have such power.