Apart from upgraded textures and new weapons, there is another, seedier side to modding. This is all well and good, modding can prolong the life of a game indefinitely - hence why more than a few people still play Skyrim pretty much exclusively six years after its release.
For nearly 25 years now, impassioned gamers have taken to the creative side of the spectrum, making custom content ranging from updated graphical overhauls, to new weapons, characters, and sometimes, a whole new game's worth of content.
