From WoWWiki
Realm First! Celestial Defender is a feat of strength achievement (no achievement points) for being the first raiders on your server to kill 
[??] Algalon the Observer in a 25-man raid (heroic difficulty). Like other realm firsts, there will be a server-wide notification when this achievement is completed.
Locations
The Algalon the Observer boss resides in Ulduar and can be unlocked by defeating the 4 watchers with their hard mode activated.
Title
This achievement rewards the title The Celestial Defender.
The title is formatted like this:
- <name>, The Celestial Defender
Patch changes
External links
Realm First! Celestial Defender
