I recently picked back up on the questline to get the Manasaber and got to this step, Lord of the Shadow Council, but could not find the quest anywhere!!! I had killed Gul’dan hoping the quest was hidden from my quest log, but did not see anywhere to turn the quest in after the kill.
After running around in Shal’Aran, I spoke with the First Arcanist Thalyssra and noticed she had the Manasaber for sale (no cost). I “bought” the Manasaber, found it in my bags, clicked on it, and got my achievement and saber… I am confused why it worked like this but hopefully this helps anyone who was having the same problem as me.