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.
Moreover, 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.
In this manner, what is the essence of clarity?
Essence of Clarity can be used through dialogue in the Scrying Pool in the Repository of Undeath, which grants 2 additional visions. This requires first acquiring three Silver Scepters from the same area and using them on the pool, granting the first of three possible visions.
How do I start Elerethe renferal?
To access Elerethe Renferal, players should take the Mulgore portal at the end of the southeast path in the Core of Nightmares. Follow the path downhill past the neutral wildlife into the spider-infested canyon below.
The quest is in two parts, the first comes from Khadgar after you kill Gorefiend, and you have to kill Gorefiend 4 times to unlock the skip to the floor after Gorefiend. The second skip comes from Khadgar after Mannoroth (afaik), where you kill Mannoroth another 4 times to unlock the skip to him.
Odyn himself is much, much easier than his two person warm-up act. Both Emerald Nightmare and Nighthold are pretty easily soloable by now, with some fights a bit tricky but not overwhelmingly so. Basically, any raid fight with weird mechanics can be a challenge to solo.