The Worldslayer Chronicle
The Rot is a strange brownish moss/lichen/ooze hybrid with dark purple, green and yellowish hues, that slowly spreads over the land, infecting and converting all organic matter in its path.
It appears nonsentient, but seeks out living matter to consume and infect.
Infected creatures start to slowly, or in rare cases very quickly, rot away. When the afflicted creature dies the Rot covers the skeletal remains and start spreading outward from the corpse.