This page explains how to complete the level 90 post-Endwalker quest "A Brother's Grief".
This quest is available if you have completed the quest Alzadaal's Legacy.
Starting the Quest
Talk to Y'shtola in Alzadaal's Legacy to begin the quest. There will be a cutscene.
Speak with Y'shtola
Talk to Y'shtola to proceed.
Speak with Varshahn
You can either go south and use the Passage to Thavnair, then go northeast to Radz-at-Han, or you can teleport to Radz-at-Han. Go northeast from the Radz-at-Han Aetheryte Plaza to where the orange arrow is pointing in the minimap. Talk to Varshahn to proceed. There will be a cutscene.
Speak with Varshahn Again
Talk to Varshahn to complete the quest. There will be a cutscene.
Next Main Scenario Quest
After you complete the quest, talk to Varshahn to accept the next quest: Sharing the Wealth.