RNC is a compression format used by Bullfrog in its games. It comes from
a tool Pro-Pack from Rob Northen Computing. This is just a standard Huffman's
compression.
It is used in Syndicate and Dungeon Keeper, probably in other games too.
It is not used in Syndicate Wars.
Note that the game engine accepts uncompressed versions of all files. You can decompress all game files, and the game will work.
Here is the site containing original RNC files extractor:
Dungeon Keeper Utilities
And here you have my version, as a project to Dev-C++.
It is compiled as Windows Console application. It has lot better
error handling than original.
Bullfrog games RNC Utilities