A Master cape of Accomplishment (more commonly known as a master skillcape among players) is a cape that symbolises reaching level 120 in a skill, at 104,273,167 experience (80,618,654 experience for Elite skills). With the exception of Dungeoneering, Invention, and Slayer, this is a virtual level, and the skill itself will remain at 99. The Master Capes of Accomplishment share the same stats and emotes as their 99 counterparts.
These Capes of Accomplishment cost 120,000 coins, and are obtainable from the same NPCs as the regular skillcapes. The NPC is located in a place related to his/her skill. These capes can only be bought and equipped on a members' server, but once equipped they may be worn on non-members servers (minus the stat adjustments, emote of that cape CAN also be performed). However, if the cape represents a member's skill and is keepsaked, it will not appear on non-members servers saying you no longer have the requirements to wear it.
Each cape comes with a special emote that can only be performed while wearing the master cape. The emotes are meant to represent the cape's respective skills. This emote is the same one as used on the master cape's respective skillcape, with the exception of the Dungeoneering and Slayer master cape, which have a unique emote[1].
^Mod Oliver. "Master Cape Poll - FAQ." 25 Feb 2014. RuneScape Forums. Would there be new emotes for each of the Master capes?
No; there would not be new emotes with the capes, they would be released with the ability to perform the related 99 skillcape emote.
^This is the number of people currently on the hiscores with the required experience for level 120 in a skill, both members and non-members.