This time is Scootaloo, Soarin and Spitfire are assigned by Sombra to conquer Cloudsdale. Then Spike and Scootaloo noticed Rumble and they approach him. Rumble is aware that King Sombra transform Spike into a dark dragon and Scootaloo into an Alicorn. Then they asked him to joined the cause of King Sombra, and Rumble is willing accepted because it's the only way to be with Spike and the CMC. So Scootaloo used her dark magic to turn Rumble into a dark Alicorn, and he wearing a Shadowbolt flight suit.

A request for chazkopa

Picture based on @mr100dragon100's comic