Why did Chrysalis invade Canterlot? To feed her changelings. After the invasion failed no one trusts changelings being within a hundred yards of them and everyone is on the watch for infiltrators. With a hive full of hungry minions what does a being who feeds on love do? She approaches the Princess of Love, and asks to help restore lost love amongst ponies. In exchange for a little quid pro quo of course.

But as bold as the Changeling Queen is, she would never have predicted who would come through her door looking for help.

Note: I started this before the Season 6 finale aired. So, all chapters should be assumed as taking place before that.