Somewhere in the everfree forest:

Chrysalis: Thanks to Starlight Glimmer stealing my hive from me it's been getting harder and harder to find love to feed off of. If I can create a magic stealing clone I won't need to depend on love for power. My new tool will do the dirty work for me.

Queen Chrysalis casts her cloning spell and a dark clone of Sunset Shimmer manifests.

Queen Chrysalis: Now my creation! Feed off of all the magic Equestria has to offer.

Onyx Sun: Why should I share the magic with her majesty? I am going to keep it all for myself and rule Equestria all on my own.

After chrysalis was turned to stone Onyx Shard began her quest to drain Equestria of its magic. Her first goal was the Crystal Empire so once she targeted canterlot Twilight wouldn't have any reinforcements.

Flurry Heart: you won't get away with this onyx shard Twilight will stop you!

Onyx Shard: Sorry it had to be this way, princess.