The problem is Darion doesn’t appear in some situations. Though, if you do these other things as well as the previous suggestion it should work. I had to work through this on my Earthen, since they were blocked on their portals and made considerable trouble completing the 20th Anniversary Quests (achievements) - see post Alyx, Alyx who the Portal is Alyx! So, if also complete the first part of the interaction of the Silithus quest (using the teleportation scroll) between Jaina and Thrall, without completing the quests. Then you can either complete or remove the Dragon expansion starter quest (to get there). You can pick up this quest again after you get the Oribos stuff completed. The only difference I noted to that described is that you get teleported straight to Oribos, not to The Maw. Then follow everything else and it should be open for you. I have also mentioned this in part in the Alyx post I mentioned, as people were having similar problems there, and therefore couldn’t complete one particular quest and the overall achievement.