This page explains how to complete the level 80 post-Shadowbringers quest "The Great Ship Vylbrand".
This quest is available if you have completed the quest On Rough Seas.
Starting the Quest
Talk to Merlwyb in the command room in Limsa Lominsa to begin the quest. There will be a cutscene.
Speak with Alisaie at Camp Overlook
Teleport to Camp Overlook in Outer La Noscea. Go to the quest icon on the map and talk to Alisaie to proceed. There will be a cutscene.
Speak with Merlwyb
Go northeast to the entrance into the U'Ghamaro Mines. Take the following route to reach the quest icon on the map:
There will be a glow around Merlwyb, indicating a solo duty. Check on her and there will be a cutscene, and then the duty will ensue.
Solo Duty: The Great Ship Vylbrand
Start by defeating the enemies that have red dots in the enemy list, then use multi-target attacks on the O'Ghomoro Golems. When the Admiral says to draw the enemies to her, make sure to pull the golems into the path of the blue cone attack markers. Afterward, defeat any remaining enemies.
After some dialogue, stand in one of the blue line markers. Then focus your attacks on the Construct 2s.
Eventually, the Construct 2s will jump to a side of the area and begin casting very large circle attacks centered on themselves. Run to the safe area on the other side to avoid these attacks. Dodge the cone attacks from the Construct 2s, and make sure that they are in the path of the blue cone attack markers.
After some dialogue, defeat all of the Bombs that appear. After some dialogue, more Bombs will appear, as well as some Grenades. Look for the targetable Grenade and stand near it while blocking the tether between it and Alisaie. Then defeat the Bombs. After that, a large number of Grenades will appear. You do not need to battle them. When Grand Storm Marshal Slafyrsyn says "stand clear", make sure to move away from him, because there will be a circle attack marker centered on him.
After some dialogue, wait for "Channel Aether" to appear at Alisaie's location. Check on it to proceed.
After more dialogue, attack the 2nd Order Rocksplitter.
2nd Order Rocksplitter
Bulldoze places a series of line attack markers across the area. Find a safe spot between them.
Tunnel Shaker is a cone attack from the front of the boss.
The boss will place a circle attack marker in the center of the area, followed by a donut attack marker around the circle, and another donut attack marker after that. Stand on one of the donut markers until the neighboring attack marker disappears, then move into the safe area where the previous attack marker was.
After it is defeated, go to Alisaie and check on the "Channel Aether" to proceed. There will be a cutscene.
Speak with Merlwyb Again
Talk to Merlwyb to complete the quest.
Next Main Scenario Quest
After you complete the quest, talk to Alphinaud to accept the next quest: Futures Rewritten. There will be a cutscene.