This is a standalone game played in normal survival (or hardcore) minecraft for java. You will need to start a new game and add this datapack before you start.
The game is altered to put you in a custom dimension at the start (purgatory).
The story goes, you were trapped and being the living thing you are you need to get back to the overworld.
To do so you'll need to explore the world and find the resources you'll need to make that happen.
the customization includes an altered starting spot, new recipes, new mechanics and books to explain all of it.
If you are looking for an extra challenge, i recommend hardcore mode.