This page explains how to complete the level 90 post-Endwalker quest "Void Theory".
This quest is available if you have completed the quest Restricted Reading.
Starting the Quest
Talk to Y'shtola in Thavnair to begin the quest.
Speak with the Veteran Radiant
Go southeast to where the orange arrow is pointing in the minimap (or teleport to Yedlihmad and go west from there). Talk to the Veteran Radiant and say Yes to proceed.
Speak with Nidhana
Talk to Nidhana to proceed. There will be a cutscene.
Check on the Destination to proceed. There will be a cutscene.
Next Main Scenario Quest
After you complete the quest, talk to Y'shtola to accept the next quest: A Satrap's Duty. There will be a cutscene.