Forget the name of the runes, but something that scribes will be able to make in legion, can only use it for 1 boss each week and lasts the entire week. A couple of guildies were discussing how it could further push a 'split run' type of idea, where 1 toon has the rune for 1st tough boss, another toon has a different rune for another boss. This would be effective until your main is geared enough to do more than the alt with a 10% damage buff, also consider people having a boomy with the 10% rune for a fight where boomkin really shines. With no tier pieces in the 1st raid (most likely, similar to highmaul) this would remain effective for a while I believe. Obviously this is a 1% very hardcore thing to consider but he talked about split clears in general anyway.
Could either make them account bound or disable them in mythic, or just let them do it this way anyway, probably wouldn't get too out of control, some world first guilds might push so many characters to have the buff for every boss for faster prog? Also do you think there will be tank and healer equivalent runes, probably will be. Will be interesting to see how these work out in progression.