Start of the game:

You are given a minute to collect resources, after which if you are in the open sky, you will be killed.
Datapack changes some game mechanics, it is written in more detail in the "Guide book".

Tutorial how install for Minecraft 1.16:

Enter or create a new world.
Install the Resource Pack:
Once the world has loaded, go to Options -> Resource Packs.
Drag and drop the “Difficult Survival In the Dark RESOURCES.zip”
into the game window.
Select the resource pack that appears.

Click on “Open Pack Folder”.
In the file explorer, press Alt + Up Arrow to go up one level.

Go to the saves folder and enter the folder of the world where you want to install the datapack.

Place the “Difficult Survival In the Dark DATA.zip” inside the datapacks folder of your world.

For Minecraft 1.19+:

In the game settings, go to Options -> Resource Packs.
Drag and drop the “Difficult Survival In the Dark RESOURCES.zip” into the game window.

In the world creation settings, click on the Data Packs button.
Drag and drop the “Difficult Survival In the Dark DATA.zip” into the game window.