This page explains how to complete the level 87 Endwalker quest "Witness to the Spectacle".
This quest is available if you have completed the quest A Past, Not Yet Come to Pass.
Starting the Quest
Talk to Venat in Elpis to begin the quest.
Speak with Venat
Go northwest until you can go south to where the orange arrow is pointing in the minimap. You can optionally go southeast to the top of the waterfall to find an Aether Current to attune to. Then at the quest icon on the map, talk to Venat.
Speak with the Peripeteia Archivist
Go south to where the orange arrow is pointing in the minimap and talk to the Peripeteia Archivist. There will be a cutscene.
Speak with the Peripeteia Archivist Again
Talk to the Peripeteia Archivist to proceed.
Speak with the Approachable Archivist
Go north to the quest icon on the map and talk to the Approachable Archivist.
Speak with the Unhurried Observer
Go northeast to where the orange arrow is pointing in the minimap and talk to the Unhurried Observer. There will be a cutscene. This one is important, so it is recommended to watch it either now or via the Unending Journey in an inn room.
Speak with Venat Again
Talk to Venat to complete the quest.
Next Main Scenario Quest
After you complete the quest, talk to Venat to accept the next quest: Worthy of his Back.