Now I have to fight my way past all the goblins outside to get back in and I'll finally get into the Underdark in one of my playthroughs.
Well ... yes, NOW you have to ...
But, if you would get into Underdark BEFORE you kill Ragzlin (meaning while all Goblins are still friendly ... or at least not-hostile) ... right down there is waypoint, just few steppes and to the right.
