You need to place the 9 planetary orbs (exclude Khull Khuum's orb) onto the pillars, in order, from Mercury to Pluto. The pillars light up in the correct order. When you first enter, the pillar for Mercury is lit. When you put an orb on the pillar, the next one lights up. (If you need help matching the pictures with the names of the orbs, see the list near the end of this document).

You can get the correct order for the spheres from the scrolls in the library in the dwarven fortress (not the evil dwarves area). If you don't want to go back there, read the book Thera's Awakening. The (almost) correct order is given there - it skips Afri. Insert Afri's orb into the middle of the list, where you think it should go.

You do not need to do anything with Khull Khuum's orb (if you even have it).

You can take the orbs off the pillars in the reverse order you put them on.

When the orbs are all placed on the correct pillars, you will know it.

Spoilers

Press shift-F8 to display current location.

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Start @(8,9)
This is it. You can't go anywhere else from here (not even back)! Place the orbs onto the pedestals in order. The lit pedestal is the active one, waiting for the next orb. See below for a list of the orbs, and the order they should be used (ignore Khull Khuum's orb, if you have it).