Finished Thunderfury the first week of Legion Launch. Wasn't anything suspicious at all and they wouldn't do that considering you can only do it once. After reading the Ashbringer hunt forum and seeing that it does indeed actually involve old content then I started to brain storm more.
The best I can think of is to talk to the NPC that you were sent to from the Silithus NPC. The Silithus NPC tells you to check out Earthcaller Franzahl who then would tell you that you need to find Krixxix in BWL. The NPC was originally in Winterspring but they moved to Mount Hyjal but as it's Mount Hyjal it's heavily phased for my rogue. Maybe other rogues can check? I'd assume you won't be able to see the NPC if you finished Mount Hyjal as my druid was able to find the NPC just fine. Another idea was that I had to bring an Enchanted Elementium to the NPC. Again, I can't check because it's heavily phased on my rogue.
http://www.wowhead.com/npc=14348/earthcaller-franzahl