The best thing about games on PC are the mods and when a game has a big enough fan base you get all kinds of crazy stuff.
Now the video below might not be of a "mod" but it is a cool example of what you can do with games on PC. Some how this guy got Manus, who is the last boss in the Dark Souls DLC (and arguably the hardest fight in the game) to battle it out with Knight Artorias.
The arena you see is where you clash swords with Artorias; how he got Manus there is beyond me*. But the epicness of this battle is ... Epic.
*Apparently there is a "cheat engine" that lets you spawn NPCs and such. The owner of the video spawned Manus in the arena and then made him hostile to NPCs.
In Dark Souls Lore Artoias does indeed battle it out with Manus - but we never get to see it.
Well now we can