After discovering the Elements of Harmony and being morphed into their Ranger forms, the Mane Six face off against Nightmare Moon's minions outside of the Castle of Royal Pony Sisters.
Seeing Nightmare Moon herself fly back into the castle, Twilight, as the Magenta Ranger, follows her and tries to use the new strength granted by the Ranger form to defeat Nightmare Moon by herself. This doesn't end very well. [link]
Overpowered and on the verge of defeat, Twilight is rescued by the rest of the Rangers, triggering her understanding of the role she plays as the uniting sixth element of the team.
An altogether unhealthy amount of time was spent staring at the screen while drawing this, so I hope all you people like it. Pony Rangers is a strange little 'verse, but I like it very much for some reason, even when there aren't giant battlemechs involved.
If you wonder why there is no Red, Blue, or Green ranger in the team, the answer is simple. RGB is an old color format. The new Rangers use CMYK.