Obtaining the Celestial Planetarium Key

Introduction
Algalon the Observer is a hard mode-only encounter in Ulduar. Algalon was sent by the titans after Watcher Loken's death activated a planetary fail-safe protection, indicating possible distress on Azeroth. Algalon's task is to analyze any systemic corruption on the planet. If deemed beyond saving, the Titans will "re-originate" the planet to cleanse it of corruption, killing all currently living organisms in the process.

He can only be accessed if least one member of a raid has obtained the Celestial Planetarium Key. Players have a one hour window to defeat him per lockout period, starting upon first initiation of the encounter. Blizzard has sardonically referred to him as "Algalon the Raid Destroyer."

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Gaining entry to the Celestial Planetarium
1. Completing the achivements I Choose You, Steelbreaker or I Choose You, Runemaster Molgeim enables one Archivum Data Disc to drop. (It is not party loot.)

2. The quest from the disc directs the player to the Archivum, located beyond the Assembly of Iron. After listening to the Archivum Console speak, Prospector Loren offers The Celestial Planetarium, asking the player to locate the Celestial Planetarium and return to Prospector Loren. Completion 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

Completing all four quests enables the quest Algalon. Returning to the Archivum Console rewards Celestial Planetarium Key.

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Detailed Achievement Descriptions

I Choose You, Steelbreaker
Defeat the Assembly of Iron with Steelbreaker as the last member alive on Normal Difficulty.

I Choose You, Runemaster Molgeim
Defeat the Assembly of Iron with Runemaster Molgeim as the last member alive on Normal Difficulty.

Knock, Knock, Knock on Wood
Defeat Freya while leaving at least 2 Elders alive on Normal Difficulty.

I Could Say That This Cache Was Rare
Defeat Hodir before he shatters his rare cache on Normal Difficulty.

Firefighter
Defeat Mimiron after activating his Self-Destruct mechanism on Normal Difficulty.

Lose Your Illusion
Defeat Thorim while Sif is present on Normal Difficulty.