Legion adds many kinds of WOW Mounts. Fathom Dweller drops from Kosumoth the Hungering in the Eye of Azshara with 10-15mins refreshes each time.  Just killing Kosumoth the Hungering will not drop anything. Instead, you have to complete a quest to unlock a World Quest. Then you will get the reward.
Mission line has no time limit and no need to complete at a time, which is similar to the quest line. It only needs to click crystals in turn on different areas. Then we can unlock the World Quest forever. The World Quest will refresh when CD refresh in 14 days. Fathom Dweller(under water), Hungering Claw and WoW Gold have a random reward which is same on all servers each CD. Once missed, you have to wait for more time to refresh.
There is no quest after completing the mission line. You need to log out and fly to the Eye of Azshara south of Dalaran. Then the World Quest will occur. Killing Kosumoth the Hungering will obtain the reward. If the mission reward is not Fathom Dweller, you just wait for the quest refreshes will be ok.