10 Weirdest Genre Shifts In Video Games
5. NieR Replicant ver.1.22474487139...

You couldn't have a list like this without NieR.
NieR became a cult classic for many reasons - its eclectic cast, its tragic story, its willingness to delete your save games - and also because of its maverick attitude towards genres.
While ostensibly an open-world adventure, NieR shamelessly flirts with other game-types during its run-time. Emil's mansion is an homage to PS1-era Survival Horror, and there are numerous instances where the game becomes a bullet hell shooter. For our money, though, the most outlandish genre-shift occurs in the Forest of Myth.
While exploring the Forest, Nier and his companion Weiss catch the Deathdream Virus. The Virus is an illness that traps people in a seemingly endless dream, which takes the form of an '80s style text adventure.
To escape the dream, Nier and Weiss have to follow the adventure to the very end. Excellently, the game fully commits to the bit - old-school logic puzzles must be solved, and movement is restricted to choosing which ordinal direction to go in.
It's a brief sequence, but a hugely enjoyable one. Especially for those of us old enough to remember fumbling around in Hitchhiker's Guide to the Galaxy...