Short story: when you reach the core you discover an altar where you can donate crowns for some anonymous religion. If the donation is sufficient a portal appears. Behind it some god/avatar/demon/whatever appears, and an epic battle begins. Difficulty should obviously match the players.
Only a general idea to inspire you dear Devs :)
hmmm i would think OOO would want to leave religion out of this
that aside what reward would be worth donating these crowns?
ooo heres an idea how about the boss's theme is based on the same principles of gate creation? the ratio of minerals determines the theme of the boss