Heroic: Thorim's Sigil

This quest asks you to defeat Thorim in his hard mode.

To activate hard mode your hallway group needs to engage Thorim before Sif disappears (before 3:00)

Unlocking the Celestial Planetarium

1. 
   
2. Complete I Choose You, Steelbreaker or I Choose You, Runemaster Molgeim, enabling one Archivum Data Disc to drop (it is not party loot; similar to Key to the Focusing Iris).
3. 
   
4. Accept the quest from the disc and bring it to the Archivum Console, located beyond the Assembly of Iron.
5. 
   
6. After listening to the Archivum Console speak, accept the quest The Celestial Planetarium from Prospector Loren. Locate the Celestial Planetarium and return to Prospector Loren.
7. 
   
8. This enables four quests to obtain the watchers' sigils, which may be performed in any order:
   
   • 
     
   • Freya's Sigil - Complete Knock, Knock, Knock on Wood.
   • 
     
   • Hodir's Sigil - Complete I Could Say That This Cache Was Rare.
   • 
     
   • Mimiron's Sigil - Complete Firefighter.
   • 
     
   • Thorim's Sigil - Complete Lose Your Illusion.
   • 
     
   
   
9. 
   
10. Complete all four quests to enable the quest Algalon from Prospector Loren.
11. 
    
12. Simply speak to the Archivum Console to complete the quest, rewarding Celestial Planetarium Key.

This appears to be a quest to defeat Thorim with his hard mode, in heroic.
It seems to be that if you can 'lure Thorim into the arena' this will make 'Sif' present in the fight...
Pure spectulation: Theres prolly a time limit or something more difficult because of this - like 'Sif' joining in? :)