https://youtu.be/_KNMLtKYRZE https://youtu.be/nNj5lVgb2Uo Karthus' Q procs Dark Harvest if the opponent's health is low enough, even if it doesn't actually cause any damage. Xerath only proc'd DH correctly, upon dealing damage to a target below 50% health.
Shield health doesn't count towards the 50% health threshold. I can have 50% health and 50% shield and when you damage me, the damage would mitigate to the shield, but would still proc DH.
I chose examples where the shield did not break, therefore the champion did not take damage. Xerath's damage wasn't procing DH when blocked, so i'm pretty sure it's not supposed to work that way.
