Rotting River

To all who used to fish in these waters: there is a blight further upstream. I have seen livestock and woodland creatures wash past in the rapids on occasion... however, yesterday I thought I'd hooked a large catch. When I reeled it in, it was a rotting corpse corrupted by some kind of blight, its skin and bones twisted and barely recognizable. As I stood there in shock, two more surfaced and floated by me. What kind of destruction is befalling these lands?