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 .