Ghost sitting on a rock in Caustic Caverns. Awesomeness.
Developer Gearbox Software paid tribute to From Software’s Dark Souls, with a little easter egg in Borderlands 2.
Found in the Caustic Caverns by Youtube user Rofl316, the easter egg’s pretty difficult to get to (understandably), and there is nothing there for you when you do. There’s a ghost sitting on a rock whose name is Solitaire, most likely a reference the Solaire of Astora from Dark Souls.
Hanging around doesn’t really do anything, but it’s still a neat-o reference for those willing to look for it. Check out the video and see if it’s worth it.