Master levels were introduced with the Trials of Atlantis expansion pack.
For each trial you complete, you will be awarded Master Abilities, chosen from specific lines tailored for your character's class. These new skills cannot be gained in any other way, and are rewards for the successful completion of the Atlantean trials. To begin your journey, speak with those in the Hall of Heroes.
Once all tasks for a given Trial are complete, advancement to the next Master Level (or Trial) is possible. With each Master Level. you will receive new abilities that will be useful in different situations, both PvE and RvR. There are 10 masterlevels available.