Disease

[General Tacticus]

I don't know about the Harvester of Sorrows, but the Demon Lord Pazzuzu also spread disease, and in his case it kills the population of the province he's in. I don't know if it also affects troops, I didn't keep him long enough near mine to find out

As for deadlier, I vote Burden of Time because it is world-wide, although the affliction gained is random.
Starvation can also be a real killer, since a unit that starve two turns in a row get diseased (100% probability). It works as PvK said, units with special survival (forest, swamp, mountain, etc.) are always the Last to starve in an army.