https://www.pcgamer.com/watch-trevor-from-gta5-run-around-in-a-bloodborne-prototype-map-found-in-dark-souls-remastered/

A modder who goes by the name Dropoff recently discovered something very interesting hidden in the data of Dark Souls Remastered—an early prototype of the Upper Cathedral Ward map from Bloodborne. Dark Souls was released in 2011, four years before Bloodborne, while Dark Souls Remastered debuted just this month—but another modder, Lance McDonald, said it's not new work.  

The data was stored in the "m99_99_98_00" file, which is what attracted attention, according to Kotaku, since the "m99" designation typically indicates a file used for testing—and where unused or leftover stuff can sometimes be found lying around. The level design found in Dark Souls Remastered isn't an exact match to what ultimately appeared in Bloodborne, which McDonald believes indicates that the level was actually prototyped during Dark Souls development, years before Bloodborne.