This datapack increases the game's difficulty in many ways.
On the first day of loading into a world with this installed, nothing too crazy will happen (at the top you can see how much time is left of the grace period). Once this timer runs out, you are not going to have a fun time.
Everything is out to get you. No matter what it is (unless it's a pig) it can and will kill you. Also, if you have diamonds or netherite during the grace period, use them, because they will disappear.
Good luck and have fun