This page explains how to complete the level 90 post-Endwalker quest "The Red Side of the Moon".
This quest is available if you have completed the quest Lunar Rendezvous.
Starting the Quest
Talk to Urianger in Mare Lamentorum to begin the quest.
Speak with Y'shtola
Teleport to Sinus Lacrimarum and go northeast to where the orange arrow is pointing in the minimap. Talk to Y'shtola to proceed. There will be a cutscene.
Survey the Area
Go to each of three quest icons and check on each Point of Interest.
Speak with Zero
Go northwest to where the orange arrow is pointing in the minimap and talk to Zero to complete the quest. There will be a cutscene.
Next Main Scenario Quest
After you complete the quest, talk to Zero to accept the next quest: Abyssal Dark.