Notice: This mod will (probably) not recieve a further update. This being a Minecraft mod really restricts what I can do.
The core of the mod is the Essences. The Essences are really powerful stuff, and you can use that to craft some powerful tools, weapons and armors. Better than Diamond! There are 3 Essences (Electric Essence, Nether Essence, Ender Essence). You craft Essences using drops from themed mobs. You'll need a lot of stuff to make Essences. Detailed description of the mod is in the Minecraft Forums thread (it's an update out of date, but most information, except the stats, still apply).
Feel free to use the mod in any modpacks. You are allowed to use it, and you do not need to ask for permission.