Originally Posted by RagnarokCzD
On the contrary Grymforge showed us that there is quite high chance that Larian will still march us on the same road no matter wich side we will choose.
(If you help Tieflings and join Halsin ... you should preffer the way through Underdark, over the Shadowlands ... yet it prooved to be dead end, since it also only lead to Shadowlands and you need to return back and head toward mountain pass.)

there seems to be an entrance to an ancient temple behind the cave-in. We can't currently go any further, but I think its likely that this is the actual underdark route mentioned. Therefore I think that we will actually have two completely different routes, not least because Larian so often hinted that 'you can have two completely different adventures'.