The Mastery skill has been made, after Jagex wanted to add super high level stuff (like mining and smithing dragon, fletching dark bows, fishing octopi, or very, very high slayer monsters). The Mastery Skill combines two skills with each other to make a new skill which is supported by these skills. To start a mastery skill, the player must have have the two skills at level 99. Not all skills can be combined with with each other, but there are many, and more may be released. You only can have 3 mastery skills at once, but you can change them anytime at Efilon, the Master in Skillmastery. He is located at the Varrock Square. The Xp you have in the mastery skill cannot be raised directly.