This page explains how to complete the level 90 post-Endwalker quest "Abyssal Dark".
This quest is available if you have completed the quest The Red Side of the Moon.
Starting the Quest
Talk to Zero on the Red Moon (accessible from Mare Lamentorum) to begin the quest.
Search for Anything of Interest
Go south to where the orange arrow is pointing in the minimap. Go to the northeast edge of the orange circle on the map and check on the Point of Interest. There will be a cutscene.
The Voidcast Dais
Use the Duty Finder to enter The Voidcast Dais. Please see the The Voidcast Dais Guide for a walkthrough.
After you complete the duty in The Voidcast Dais and exit the area, there will be a cutscene.
Speak with Zero
Talk to Zero to complete the quest.
Next Main Scenario Quest
After you complete the quest, talk to Y'shtola to accept the next quest: The Dark Throne. There will be a cutscene.