It’s taken eight years, but a group of modders have worked out a way to create custom levels for Dark Souls, and to prove it they’ve imported... a classic Half-Life map.
As explained in this Reddit thread, the hold-up has long been down to not just Dark Souls’ use of Havok physics, but the fact that “the file format and stored data change drastically between game to game”.
This has been overcome by modders Meowmaritus and Horkrux, the former working out how to import models (albeit without physics), the latter discovering that an old public release of Havok let them get collisions working. Which, incidentally, limits this discovery to Dark Souls 1 Prepare To Die Edition & Dark Souls 2.
It’s still very early days as far as this is concerned, and given how long it took to just get this far, it might be a while until it’s relatively easy for modders to just go importing whatever they want. But it’s progress!
To show the advances in action, here’s classic Half-Life 1 deathmatch map Crossfire imported into the original Dark Souls.