"Obscuro's Overhaul changes the levels of some NPCs in the game, fixing them at certain levels, meaning that when you are low level the world is a mean and unforgiving place and that when you are high level the place is still mean and unforgiving, but due to some minor adjustments to the spawns you'll still find rats and cute low level monsters to drop kick around the forest. Because it changes NPC levels that are fixed when the game starts up, for Obscuro's mod to work you need to start the game from scratch with it activated."
Read on over at Bit Tech.