Defeat Algalon the Observer in 25-player mode.
Algalon is the last, and most difficult encounter inside Ulduar. You can only fight him once you've completed several other hard mode versions of other bosses. Here's the run down of which hard modes you'll need to do:

• Iron Council - Either with Runemaster Molgeim or Steelbreaker last. This will allow someone in the raid to loot Archivum Data Disc which will begin the quest for Algalon.
• Hodir
• Freya
• Thorim
• Mimiron

Each of those bosses will drop sigils and you'll need to collect all of them to complete the Algalon quest and get the Celestial Planetarium Key. Please note you'll have to defeat Yogg in order to kill Algalon!