Thinking about acquiring a GRUSH,
but how does it work BAAL wise?
After one hits 20/40/60 Throne unlocks?
Or how does one baal?

One does not simply unlocks Throne Room.

The prereq for completing the Baal quest is competing the ancient quest. So someone has to be level 20/40/60 respectively AND HAVE THE ANCIENTS QUEST COMPLETED. Then you have to have that person be in the chamber of secrets (or whatever Baals secret room is called) when Baal dies. Then everybody in the game finishes the quest and can move onto the next difficulty level.