Here's the developer, Nate's description of the game:
Pokémon Apex is developed with an adult Pokémon fan in mind. The game contains some adult dialogue (some swearing) and themes (death, abandonment), and some minor horror elements. Furthermore, the game is intended to be somewhat difficult, meaning that you will need to think about your strategies and make conscientious choices about your team in order to progress. Playing through the game haphazardly will get you into trouble.
Also, this game is for people who prefer to take their time in games to explore their environment and uncover hidden details. There are lots of small details hidden throughout the game, as well as secret areas and items that reward meticulous players.
The game's still in development, but you can download it over here to try it out!