Mastery Skill

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.

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. Note, only a maximum of 26,068,862 XP is added from each skill. This means, that you need 26,068,862 XP in both skills to get level 99 in a mastery skill.

Skills which can be combinied
(Feel free to add some...)
 * Dungeoneering level 99 can be combined with Summoning, but level 120 gives extra monsters to summon.
 * For some reasons, there is no skill which can be combined with the Dungeoneering skill. This is because Dungeoneering stops at level 120 (instead of 99). There would also be no use for it.

Trivia

 * The name of the skillmaster Efilon means Nolife if read backwards. This could be, because this guy has level 99 in all mastery skills (which means he has 26,068,862 XP in all skills, which is quite a lot).
 * The best combination of mastery skills for PvP is the "Assassin, Blademaster and Berserker" combination, often called an "ABB" pure. With these, a player is almost invincible, as he can hide and deal out a lot of damage at once. There aren't much ABBs out there, as training all those´ skills to 99 (especially agility) takes a lot of time.
 * ABBs can now be countered with the addition of the new Tracker skill.