Mods allow for some interesting crossovers and deep-dives into the files and assets of games, but rarely do we see such a hodgepodge of IPs facilitated by clever ripping and loading.
While not a mod per-se, it is possible to access a level from Bloodborne, a PS4 exclusive, ripped from the Dark Souls Remastered files through Grand Theft Auto 5 on PC.
If that isn't a scattershot concept, I don't know what is.
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The recently released Dark Souls Remastered, beyond fixing the game's awful performance on PC, also happens to have some odd file-junk laying around its directories. It is hardly rare for release versions to still have cut content, debugging tools and additional assets which aren't present in the game kicking about the installation folder - but this case is rather unique.
For whatever reason (I'm keeping fingers crossed for a Bloodborne PC port) the PC version of Dark Souls Remastered contains a few files associated with Bloodborne. Bloodborne is a Souls-like game from the same developers that was released on the PlayStation 4 in 2015. How some of its assets wound up in the PC version of the remaster of a 2011 game is beyond us - maybe it was used for testing purposes?
In either case, ripping the rather nondescript "m99_99_98_00" map file from the game directory and loading it up in GTA 5 will result in us being able to explore a half-complete mono-textured version of the Upper Cathedral Ward. While the novelty of traipsing around a location from Yharnam as Trevor or Franklin is worth enough as it is, there is more to unwrap here.
Based on Kotaku's report, it seems that Dark Souls/Bloodborne dataminers in the past have uncovered that the map layout which later became Upper Cathedral Ward actually dates all the way back to the development phase of Dark Souls, the remaster of which carried this file.
This would make sense, as this means that Upper Cathedral Ward was cut content from the first game and was likely used for testing and it served this purpose in the remaster too. Incidentally, Dark Souls Remastered also contained level files for Bolataria from Demon's Souls.
As for opening Dark Souls map files in the GTA 5 engine, the modder who pulled this off explained that he used software to convert the files to a format which was compatible with RAGE but didn't alter the content. This allows one to load Upper Cathedral Ward and drive around with a GTA 5 vehicle for one of the craziest gaming experiences possible.
In case you ever had the urge to play a prototype version of this Bloodborne location, then not only do you have oddly specific desires but your pleas have been answered too.