The mod team known as Datehacks, who have previously released a Dark Souls hack which allowed you to take control of bosses within the confines of a boss arena, have now taken their work to the next level with a new release which allows you to brutally mame and slaughter NPC's to your hearts content in any level of the game. 

The mod gives you the ability to play as any boss character that you choose, complete with that boss' power and ability, but the new Age of Fire mod lets you take these bosses throughout the entire game. Scale the flying buttresses of Anor Londo, bright light to the Abyss as Gwyn, or conquer the Darkroot forest as Artorias.

[quote]Naturally, you'll be able to use any of that enemy's attacks as well. I can finally live out my lifelong dream of becoming a wheelie skeleton. [/quote]

While the mod is still a work in progress, but the mod promises that even with giant characters like Seath the Scaleless, you'll be able to fit into tight spaces thanks to some modifications to the camera and other techniques. The only limitation is that you wont be able to seemlessly travel between maps, due to limitations of the game engine, but you will be able to select a map, then complete it using the boss of your choice.

Of course, this mod will only work in single player, and it is only for the Prepare to Die edition, not the newly released remastered version. 

You can find out more about the project and follow its progress on the reddit thread here.