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Worth: 30G

For this achievement as stated you need to unlock every WWE Championship title and have them included in your belt case which can be found in your locker room.

Here's a short list of the belts needed and a quick explanation on what's needed to obtain each of them:

Smoking Skull Belt
For this belt you need to win the Royal Rumble event during the WWE season mode. However you can only enter the Royal Rumble provided you're not already that brands champion.
Million Dollar Belt
For this belt you need to complete all of the "Legend Challenges", these can be accessed via the TV on the bar in your locker room (You'll need to unlock the different WWE Legends before you can access these challenges).
Hardcore Belt
For this belt you need to complete all of the "Superstar Challenges", these can be accessed via the TV on the bar in your locker room.
World Heavyweight Championship -
For this belt you need to either win the belt or (if you're the current champion) successfully defend the championship at the PPV "Wrestlemania" in season mode.
WWE Championship
For this belt you need to either win the belt or (if you're the current champion) successfully defend the championship at the PPV "Unforgiven" in season mode.

The following belts are already included in your belt case:
United States Championship
Intercontinental Championship
WWE Tag Team Championship
World Tag Team Championship
Cruiserweight Championship
Women's Championship

Once you've unlocked all of the championships you need to use the computer on your desk to enter "WWE Shop" for the achievement to unlock.