1. Start by ranking an Assassin Apprentice up to level 10.
2. Once you reach level 10, you can then assign this apprentice to an Assassin's Den.
3. Locate and complete the Master Assassin Mission that opens up.
4. Once completed, you will need to continue leveling your apprentice up to level 14 with 15500 XP.
5. Once you have reached this mark you will unlock part 2 of your Master Assassin mission.
6. Completing part 2 of your Master Assassin mission will level the apprentice up to the rank of Master Assassin.