The Unstable Prism

As of legion he is in old Dalaran in Northrend. quickest way to get there is to use portal in Legion Dalaran to vale of eternal blossoms and use the Dalaran portal there.

This quest can be obtained from the [item=129928] which drops once per event by doing timewalking dungeons.

Edit: The quest can be turned in to [npc=98690] during the Wrath Timewalking event and 24 hours after it's ended.

Check your quest completion during the event.

The Unstable Prism

Legacy Script - Wrath Classic
/script print(IsQuestFlaggedCompleted(40173))

Modern Script - Retail WoW
/script print(C_QuestLog.IsQuestFlaggedCompleted(40173))

• 
  
• TRUE if completed.
• 
  
• FALSE if you have not yet completed it during the current event.
• 
  

This is a basic script to check quest completion. Run it from your chat window as you would any other command in the game. Similar, albeit more advanced scripts, exist to check for NPCs killed each week such as the World Bosses in Draenor.

You can also substitute the 5-digit quest code above with other quest codes found on Battle.net and here on Wowhead to check their completion status.