Here is artist’s description:
 
Well somebody’s gotta defeat the Dazzlings in the battle of the bands (>>1091440). So the Rainbooms are literally working together to defeat the Dazzling’s fusion along with the newest member of the Mane 7, Sunset Shimmer.
 
“You’re never gonna bring us down! You’re never gonna break this part of us!… We’re here to let you know, that we won’t let it go! Our music is a bomb and it’s about to blow!” -Osumare Melody
 
Osumare’s design is based on the alicorn the Mane 7 summon when they’re about to defeat the dazzlings. When you think about it, the alicorn is kind of like a fusion itself. It’s made out of the elements of harmony plus another magical element of harmony so it’s similar to a fusion.  
The name “Osumare” means “Rainbow” in African Yoruba. I wanted to give their fusion a rainbow name along with a music name. Because if I literally named her Rainbow Melody, then it would be really confusing, especially since I already made two fusions with the name “Rainbow” in it. So I decided to be a little bit creative with the name.

By your powers combined!
 
If this is the humane 7 as a fusion, it still looks better than if the fandom fused into one self-hating entity.

@Background Pony #F349  
It’s just a cheap tactic.

I guess she’s Stronger than You…

Osumare Melody she is not a hero♫  
Gonna take the movie quality to zero♪

Yeah, that looks like the kind of mess Hasbro’s toy department would come up with…