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. You start in elysium (a heavenly place) and search out the things you will need to rescue Todd. One of the locals who is trapped in the abyss.
There are 9 custom realms that take you through the planes of the afterlife where you look for the imprisoned Todd. (The nether and The end are also available, though the normal overworld is not)
There are a few quests and custom vendors to help you along your way.