(Copied from the artist’s description)
 
Rainbow Dash kept her grip very firm around Rarity’s left leg, until it fused together with the latter’s right leg, morphing into a strong, slender mermaid tail, purple in color and softly scaled in texture. The fashionista girl was holding her breath all this time, but she kept sinking and sinking, pushed by her multicoloured friend. Rarity could not hold her breath any longer and, in panic and surprise, she Lets it Go. Her last breath is exhaled, but to her surprise, she found herself breathing and alive, thanks to her new gills. Too bad she doesn’t notice it because she is understandably more interested in her lack of legs and feet. She does what every single warm-blooded human would do in her situation: scream.
 
Dash is satisfied and looks at her handywork. Now there is only two friends to convert and share in the curse: Applejack and Fluttershy.

@Background Pony #AF40  
I for one welcome our new Russian overlords.
 

@Background Human  
After looking at the whole series up till now, I believe this was all part of Putin’s plan. He put a scrambler on the portal to send Twilight to the bottom of the sea instead of CHS, turning her into a mermaid instead of a human, and she in turn cursed Pinkie, Pinkie cursed Rainbow Dash because she’s, well, Pinkie, and I believe RD did Rarity because I think she’s snapped and has decided that if she can’t legs, no one can (and the way things are looking, she’s about to do AJ the same way too for the same reasons).
 
Meanwhile, with it’s human defenders stuck under the sea along with the pony princess, leaving Equestria weakened as well, Putin will invade CHS and by extension Equestria, then claim that they were “liberating oppressed Russian ponies by annexing magical horse-land.”

I’m a bit concerned by Red October back there. Is torpedoing a mermaid considered a war crime?

Something is fishy around here.