This page explains how to complete the level 90 post-Endwalker quest "Once More unto the Void".
This quest is available if you have completed the quest Buried Memory.
Starting the Quest
Talk to Varshahn in Radz-at-Han to begin the quest.
Speak with Zero
Go to the nearest Aethernet Shard and use it to go to Kama. Then go southwest until you can go west to where the orange arrow is pointing in the minimap. Talk to Zero to proceed. There will be a cutscene.
Speak with Varshahn
Jump down from the walkway and go north to the Aetheryte Plaza. Use the Aetheryte to go to Meghaduta. Go northeast to the quest icon and talk to Varshahn to complete the quest. There will be a cutscene.
Next Main Scenario Quest
After you complete the quest, talk to Estinien to accept the next quest: A Cold Reunion. There will be a cutscene.