Dark Harvest feels weak. I know a good way to buff it (no BS)
Currently dark harvest is pretty meh (compared to the 8.22 one) and you gotta work a lot harder for stacks but the DMG per stack is not impressive.
Now on the other end if you boost it too much it becomes a 50% HP execute.
Since we don't want that either how about you leave it a 8 DMG per stack BUT give it bonus DMG against shields.
It would fit thematically as it's intended for the assassins. A lot of ADC-s and Mages get shielded while you do your burst down combo.
If Dark Harvest did it's current meh less than 400 DMG (20 stacks) at 30 minutes it won't do much.
#BUT imagine if we gave the keystone a 2% bonus DMG VS shields per stack.
Bam suddenly it starts to feel pretty good on assassins cause of that ridiculous 400-1000 HP shield barrage that regularly gets dropped on a priority target while you are doing your ALL IN.
With the 40% extra shield DMG at 20 souls your DH now does a really decent amount of DMG to shields (not 400 but 560) The mastery will now serve a good purpose as it would obliterate the shields and allow you to finish your kill.
On the other hand if the enemy supports were quick enough to shield before the target dropped to 50% HP they are rewarded by preventing the DH proc and making the assassin waste his DMG on the shields and die.
I don't like the current DH so please do think about implementing a shield buster effect on it.