JITD's Aggressive Mod changes Dark Souls enemies so they attack you sooner and pursue when you try to run away.
Dark Souls, at its core, is a game about education. Progress is often dependent just as much on the player being patient as skilled. Taking the time to learn where enemies are and how they behave can be just as valuable as high stats and a good sword. That said, one Dark Souls fan, going by the moniker JITD, has crafted a mod for the PC version of the game designed to render your past Dark Souls experiences null and void.
The Dark Souls Aggressive Mod, put shortly, enhances enemy's desire to poke you full of holes with sharp, pointy objects. The actual difference this can make in the game is fairly dramatic. For instance, whereas in the past enemies wouldn't approach you until you came within a certain distance, the mod erases "invisible barriers" so that they'll start toward you the second you enter their line of sight. Enemies will now also pursue you with greater enthusiasm than they did in the past, making it less viable to simply sprint past them to safety.
What this amounts to is an experience where you'll need to be on your guard even more than before and will pretty much have to fight everything you encounter. The changes, in turn, add up to a game that is fundamentally the same, but altered just enough to feel fresh and challenging for veterans. If you're interested in giving the mod a try, it's available as a free download.
Source: Insane Difficulty