Mastery Skill

The Mastery skill has been made, after jagex wanted to add super high level stuff (like mining/smithing dragon, fletching dark bows, fishing octopuses or very very high slayer monsters).

The Mastery Skill combinates 2 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 combinated with with each skill, but there are many, and there may be released more. 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.

Xp Formula
Mastery skill XP=(Skill 1 XP+Skill 2 XP)/2-13,034,431

So for example, you have one 99 Skill with 15,500,340 XP and one Skill with 17,300,500 XP, the your Mastery Skill XP would be:

(15,500,340+17,300,500)/2-13,034,431=3,365,989 which is level 85.

Trivia
There is currently no Mastery Skill for Herlbore.