Essentially, it's my black hearted Harvester of Sorrows with 25708 experience, with one assassination kill to a Seraph (he had the experience before the kill). Sadly I didn't save the turn files before I noticed it in the HoF. Weird bug? I don't think he killed that many people from disease yet, he's only been around a couple turns. Maybe there were some high-xp villagers around...
This is one of the main reasons Dominions_3 needs to have a method which allows players to disable/enable spells and items. Thus if something is broken or unbalanced in the game players can remove this before they start.