Whispering caverns questline impossible to finish. Bugged quest

I did all the new quest related to the artifacts and so but there is this quest that is impossible to complete. The quest tracker brings me to the whispering caverns but when I enter the zone there is no more tracker. Cannot talk with the related npc because... there is no NPC. When I go to the live server and take a look to the completed quest, the game literaly tells me that I've alredy completed that quest