Spoilers to the Endings

These are all the endings that are possible within the scope of the finished game. There may be things you need to do to solve each ending, and they will be listed below. Any items needed for the ending MUST be found before going into the Underworld for the final time...or else you may be doomed to one of the lesser endings!

Ending in Chaos	Your adventure will end in Chaos if you fight AND kill Sariloth. He will fall and the world will be taken over by the Chaos of Thorolis - and the brothers will die in the ensuing lack of order.
Ending in Order	If you join Sariloth, the brothers along with Sariloth will defeat Thorolis, and a new world order will appear. But the brothers are mere mortals, and upon their deaths, Sariloth will encapsule the world completely in crystal, thus achieving a completely ordered state.
Good Ending	The remaining brother or brothers (assuming one or two have died along the way) will have found and repaired the broken StarGate (by using the Object of Hope of course!) and will take that StarGate to Thorolis and banter with him. Upon his defeat, you will return to Sariloth, and defeat him using the Chaos Orb (which you DID get somewhere in the ending, heh.)
Best Ending	All THREE brothers have survived and will have found and repaired the broken StarGate (by using the Object of Hope of course!) and will take that StarGate to Thorolis and banter with him. Upon his defeat, you will return to Sariloth, and defeat him using the Chaos Orb (which you DID get somewhere in the ending, heh.)

Which ending is actually the best is up to you, the adventurer. I have only listed them as they would appear to the three brothers. Any questions or comments on these endings, feel free to email me.