Most damned souls are devoured by demons shortly after their arrival in Hell. This could almost be thought of as a small mercy, because Hell is, well, Hell.
The ones who manage to survive by preying on the souls of those weaker than themselves become twisted and monstruous, until they are indistinguishable from the demons that once hunted them.
Agdistis, the game's main character, is one such soul. Their human life was cut short by a great injustice, and as a result, they harbor a deep grudge against a world that they believe failed them in every possible way. Their ambition is simple: To reach the top of the White Tower, cross the treshold into the living world, and CLEANSE IT WITH UNHOLY FIRE.
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Some people were curious about the choice of name for this character. Basically, they chose to name themselves after a mythological androgynous deity because they don't identify as either male or female (That's also why I've been using the singular they to refer to them).