Treasures of Aht Urhgan Mission 15
The Black Coffin
After your cutscene, head to the Dvucca Isle Staging Point through the Runic Portal. Then, go West to G-9 and zone into Arrapago Reef.
Travel through the Reef North to H-8 and find the "Cutter". Approach it for a cutscene. After the cutscene, you'll need to enter a BCNM through the Cutter.
Everybody (even those helping who have completed this before) need an Ephramadian Gold Coin to enter. If you've done this mission already, you do not need to fight Jazaraat again to receive another coin, but you do need to check his headstone.
This BCNM behaves similarly to Assault areas. As one member enters, all nearby members will enter as well, and all food/buffs will be removed upon entering. During the fight, your party will be joined by Gessho, the NPC Yagudo from this mission line.
There are two parts to this BCNM. You can buff upon entering.
First, five Fomors will appear and attack your party. They are: two red mages (RDM), two rangers (RNG) and one corsair (COR). Sleepga can be used to neutralize the Fomors.
Once you've defeated all five, another set of five will appear. This time there are two rangers, a corsaid and only one red mage. With them is the Ashu Talif Captain. Watch out for his Vulcan Shot, which does about 700 damage in the area of effect (AoE).
The goal here is to bring the captain down to 20% HP, so focus on that if you can. Once complete, you'll be returned to Nashmau.
Return to Whitegate and speak with Naja Salaheem to continue.