Players must defeat Nythendra first to gain access to the Core of the Nightmare, which holds portals to other Nightmare locations. The raid may attempt the four bosses linked to the Core of the Nightmare – Ursoc, Il’gynoth, Elerethe Renferal, and the Dragons of Nightmare – in any order.
Likewise, can you skip to xavius?
If you finish the quest you most certainly only need to kill Nythendra, then you can use the Iris to do Cenarius and Xavius; skipping the aforementioned bosses exactly as described.
Also to know is, where do I turn in Emerald Nightmare piercing the veil?
When you complete The Emerald Nightmare raid, Malfurion is no longer trapped in the Iris so you are not able to turn in the The Emerald Nightmare: Piercing the Veil. You will have to enter a new instance of the raid, either with a different group, or on a different difficulty, to turn in the quest.