Hey Bridleway! Did you see the pic I made for you? :)
Hey there!
Of course! Forgive me if this is a longer explanation then you wanted. I just get really excited while explaining my stories.
Okay, the animatic I posted consists of flashbacks when Flash and Twilight divorced. He just walked out on her. It is not explicitly stated why he left but these are my thoughts.
____________________________________________________________________
Flash is a member of the royal guard- one of the highest ranks (a personal escort and bodyguard to the princess). When he married Twilight, he was personally stationed to her.
One night, when Midnight was roughly 8-years-old, the castle was attacked. Apparently some ponies were unhappy with Twilight's idea to bridge kingdoms with other creatures and planned to overthrow her. Luckily no one died, but Twilight was seriously injured.
Flash realized that as royalty, his family would always be at risk during war and the like. Fearful that he didn't have the strength to protect them and not wanting to see them die, he broke up with Twilight and ran away. For years, Twilight sent him letters, hoping he'd respond or come back but he never did. Midnight is still in rage over what his father did.