Everyone knows Dark Souls (opens in new tab) is hard, but that doesn't mean that experienced players aren't up for making it even trickier. Jack wrote a guide to doing just that last year (opens in new tab), and a new mod called The Scorched Contract would slot in nicely. Every time you beat a boss you gain a debilitating debuff, making the next battle even more of a slog.
Beating lower level bosses grants less crippling effects, while the debuffs you get from harder bosses "could be simple challenge runs on their own", according to the mod's creator HotPocketRemix. After every boss battle, you'll get an item that describes the debuff—although some of them are "cryptic", which makes it even more difficult.
The mod's description also says that defeating bosses will "change the world of Lordran", which means certain doors will lock, or enemy positions might change.
If you fancy it, then go to the mod's Nexus page (opens in new tab) for installation instructions. HotPocketRemix says they'd consider adapting the mod for Dark Souls Remastered (opens in new tab) in the future.