You wake up in a chamber--more like a cell, really. Your memory is hazy. You might have been thrown into prison, wrongfully or otherwise. You might be suffering the consequences of your choices in the afterlife. You might be experiencing a nightmare, or an altered state of mind. You might have been trapped here by a cruel magician, or a crueler god.
Welcome to Nucleus, your only oasis of certainty in this plane of Primal Chaos.
Your ultimate goal is to escape this plane by finding the dungeon that exits to your home plane of existence. You may, in the process, find out who the benefactor of the Nucleus is, why these random dungeons exist in the Mist, and even how you came to be in this plane in the first place. Or you can just concentrate on escaping with as much loot and experience as possible. Your choice.
Dungeons, monsters, treasure etc. are for the most part generated randomly using
donjon RPG tools. There will be exceptions, notably for unique enemies and treasure.
Dying in this plane is not permanent. You may choose to revive at the Nucleus, but your character will permanently lose a point of Constitution, and will also lose all treasure and equipment. There may be rare treasure rewards that prevent one or all of these setbacks, or you can dungeon delve with companions who can revive or resurrect you.
Alternately, you can choose to reincarnate (create a new character) upon death. You retain access to your previous character's vault, but restart at level 1. If you have unused Hero Points from your former life, then you may keep those Hero Points (to be used as per Pathfinder rules) or add them to your new character's ability point pool.
More details about character creation and rules for play can be found on the game forum. Happy adventuring!