1- the item that starts this questline is looted on Dracaryz in Isle of conquest  (3% chance per kill)

 

            

 

2- Meet Ner'Zhul Spirit in the mall to finish the quest.

 

3- He will ask you to bring 30 demonic essences 

4/5/6/7/8 - You need to bring 30 essences of each type in order from gm summons event 

    

9- now you need to collect all 3 legendary pendant pieces 

the first one is located in dalaran crater

10- the second one is located is located  in Fire Plume Ridge inside Un'Goro Crater​​​​​​

11- the third and final one is located Demon Fall Canyon in Ashenvale

12- now what's left is asking Ner'zhul to restore the pendant for you and choosing the right version !

  

after that you can upgrade it to reach rank 20 like the other legendary accessories .