Algalon

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:
   
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   • Freya's Sigil - Complete Knock, Knock, Knock on Wood.
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   • Hodir's Sigil - Complete I Could Say That This Cache Was Rare.
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   • Mimiron's Sigil - Complete Firefighter.
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   • Thorim's Sigil - Complete Lose Your Illusion.
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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.

Go look at the items themselves. They are quest items, and flagged as party loot. Anyone with the related quest will be able to get a sigil for that boss, provided you run the hard mode.

Information from Algalon: He will DESPAWN after ONE HOUR of attempts, and will be gone for the rest of the WEEK. Blue post confirmation below.

http://forums.worldofwarcraft.com/thread.html?topicId=16473611256&pageNo=3&sid=1#44

One hour per week.

Edit: He sounds absolutely badass. http://www.youtube.com/watch?v=r2MwblTnb-g