The Dark Phoenix mount is a reward unlocked by Guilds who have completed Guild Glory of the Cataclysm Raider, which is to clear Blackwing Descent, Throne of the Four Winds and Bastion of Twilight with 8 Guild players in the group for 10man and 20 Guild players in the group for 25man.
Once your Guild has this unlocked, anyone who is Exalted rank with the guild can purchase the mount, regardless of if they helped with the Guild Achievement or not.
If you’re in a guild that has completed the mentioned achievement and you are Exalted reputation with your Guild, the next step is to locate a Guild Vendor. These are found in the majority of Horde and Alliance major cities and the mount can be purchased for 3,000g
If you aren’t Exalted with your Guild, then luckily it doesn’t take long to achieve, especially if you purchase the Renowned Tabard if available or even the Illustrious Tabard, as these will give you bonus rep. From here just complete quests and/or dailies and you will get a good chunk of reputation per quest you hand in.