The Hardcore Ruleset is an extension to Neverwinter Nights, written by William Bryan (better known as Archaegeo Neosophy in NWN circles) and others.
Bioware advertised NWN as a game that's so close to real roleplaying that you could smell the dice, rulebooks and snacks. Most of the things they advertised were fulfilled, but some still felt the game was too close to Diablo II. The thing was, of course, that NWN had the potential to become a really rigorous RPG experience through the wide scripting and editing support - there was the framework to make most of your wildest dreams come true. Thus, people worked on the Hardcore Ruleset to make the game more like the real deal.
Hardcore Ruleset adds a lot of features that make the game even closer to the pen-and-paper D&D. It is simply a set of ERF files that you can import to an existing module (though you need to make careful changes to the content of the module and enable features that you actually need, because the modules need to be made with the hardcore rules in mind!)
Here are some of the things supported:
- Support for demihuman subraces, something that was there in NWN but didn't have anything other than a cosmetic value. Now, subraces actually matter in the game play.
- Better XP support (For example, fairer distribution of XP, "trainers" you need to consult in order to level up so you won't be able to do that "on the field", and XP is not given for effortlessly killed enemies or near-impossible to kill creatures (DM should award the XP in that case))
- Bleeding and dying: Under 0 HP, bleeding and stabilization as described in the rules, death only on -10 HP, and passing to the Fugue Plane if not resurrected by deities.
- Resting: Only heals 1 HP, optional requirement of food, resting only once per 24 hours.
- More logical trap/secret door searching system.
- Many spell changes (such as material components) and some changes to animal companions/familiars.
- Wandering monsters. 'nuff said.
The rules can be turned on and off, and indeed, features that don't fit the module should be disabled or finetuned to make a balanced experience (for example, XP system should be tuned in any case). The features can also be enabled and disabled when the module is running, using the DM's tools.
There's two forks of the code: HCR, the original version, is at 2.0 and will be the final version, and development of HCM ("Hardcore Modular") has begun and first versions are out. The reason for reimplementation from scratch was the original's tendency to become unmanageable and the growth of build times - the new system is modular, consisting of a very light core, and separate modules to bring necessary features. This is important because the old HCR was basically a gigantic lump of stuff to be imported.
Servers that run modules using the hardcore rules have [HCR] in the title.
Hardcore Ruleset homepage at the moment of writing: http://nwn.darkemud.com/HC/