Heroic: 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.

Finally got this figured out.

Heroic Iron Council on Hard Mode will drop a quest item like Sapphiron.
This seems to be the quest: Heroic: Archivum Data Disc

It seems that you then get Heroic: The Celestial Planetarium* where you are required to check out the door, then 4 more quests open up, which require you to get 4 sigils, from 4 bosses on Hard Mode.

Freya - Heroic: Freya's Sigil
Hodir - Heroic: Hodir's Sigil
Thorim - Heroic: Thorim's Sigil
Mimiron - Heroic: Mimiron's Sigil

After doing so, Heroic: Algalon will open up, and allow you to obtain the key to Algalon.

After you defeat Algalon, one player may be able to open up a new quest to obtain 1 of 4 epics.
Heroic: All Is Well That Ends Well

Everything in italics is my assumption, everything else is confirmed by other sources, whether it be players or from other quest pages.

*Fits the storyline how you check out the door, you go back to her and tell her it's locked, and she replies something like "Well I can think of a way but it requires blah blah blah."

Algalon the Watcher is a hard-mode only boss in Ulduar - this quest appears to be the followup from doing all the qeusts to do the other Watcher Bosses (Thorium, Freya, etc) with their hard modes, and allows you to fight Algalon 25-man version.