This mod (which is still a work in progress) adds lots of new adventure elements to Minecraft. It contains:
- New ores to fill up the caves
- New mobs to populate the world
- New dimensions to travel to
- New bosses to battle against
- And more!
The mod is themed around special bones that can be used to craft powerful tools and armors. These bones are crafted in a way that allows a structural progression of the game. Basically, you can't get the amazing stuff until you have the stuff that comes before it (unless you cheat, of course).
The mod is also open-source, and the code is here.