Twilight tries to negotiate with King Sombra in order to get him to stop trying to enslave the crystal ponies. He agrees to let the ponies live their lives normally and free under one condition. Twilight must become his property. His slave to submit to his every desire. Surprisingly, she makes the sacrifice and accepts this deal and what starts out as a way to buy her friends some time until they can deal with Sombra once and for all, turns into something more serious...

...something that could means abandoning her friends.

Edited with the help of IRPony. For the love of God, don't click the source in public!