After playing around with Epidemius for a bit I finally hit a tally of 20+ (Epidemical) which adds the debuff Greater Plague Afflicted to all enemies that have a plague. It causes 20-40 damge per second while they are in melee. The issue is that if you have a plague, the debuff gets applied to your own troops, as well. My Exalted Plaguebearers with stats around 60 and 30% physical resistance then died fighting a few lowly marauders from a garrison which left me a sad derp. 

He deals damage to his own army when they are plagued and he has tier 2 or 3 of his faction ability.

Grandfather has forsaken us.