Just an idea. Might be a good addition to Thaumcraft and probably already has been done by someone.
Any death in a area adds to a hidden score that accumulates over time. Never actually drops, probably. It generate bad karma/effects in the area based on that score. It would have to extremely high to get the effects. From things like more powerful mobs that are harder to kill. To a taint like stuff that spreads through the the chunk and possibly nearby chunks. Possibly even instability effects that are save wide(these would require a way to lessen the kill score, don't much care for making the save unplayable)
Was thinking this would be a relatively high score. Like a 200 deaths in one chunk in a day might set it off. Or 10,000 deaths in a chunk over time. Whatever. The whole purpose is to discourage mob farms or any sort. PVP arenas should be a issue. The idea is to make the score high enough that it Would take automated systems to set it off, and even those would need to be going full bore.