The Celestial Planetarium

Quest chain:

1. Archivum Data Disc - Started from Archivum Data Disc - Dropped by killing Molgeim or Steelbreaker last in the Iron Council ecounter.
2. 
   
3. The Celestial Planetarium
4. 
   
5. • Hodir's Sigil - Defeat Hodir on his hard mode
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   • Thorim's Sigil - Defeat Thorim on his hard mode
   • 
     
   • Freya's Sigil - Defeat Freya on her hard mode
   • 
     
   • Mimiron's Sigil - Defeat Mimiron on his hard mode
6. 
   
7. Algalon - Recieve the Celestial Planetarium Key which allows the entire raid acces to Algalon once Yogg-Saron has been killed.
8. 
   
9. Kill Algalon to be able to get All is well that ends well.

Once you obtain the discs and begin this quest, Make sure you return to Prospector Loren before you continue killing bosses, otherwise you will have to wait till next week to turn in this quest and continue the chain.

I had this quest in my log for ages, so when I went to finally turn it in, [npc=33956] was not there. Apparently he disappeared when 10-man and 25-man modes were merged, making the sigil questline impossible to complete.