If anypony owes somepony a life debt, it's Rarity for that one time when Rarity had wings, and then they got ruined, and then Rainbow Dash saved her from plummeting to her doom. After a lightning strike renders Dash unconcious and sends her falling over the edge of a cliff, Rarity is the only one around to save her.
I did this for the artist training grounds at EQD, where the prompt was to draw a pony following the noble dragon code. I experimented with my process this time around to try to focus on value and contrast. The story may have worked better as a series of panels, but I wanted to try and convey as much as I could in a single image.