The idea from Halo Wars 2: "Awakening the Nightmare" DLC.

Somewhere in the edge of Equestria's border, Queen Chrysalis's Changelings are expanding their territory into the unknown regions, after their fail attempt to take over Canterlot.
Planning to expand the hive and gather resources to try invade Equestria once again.

While expanding, they stumble upon a ancient sealed underground temple from early Equestrian empire era. Having no a clue what the Temple was build for, they decide to open it and salvage everything what they can find. Accidentally releasing a ancient swarm from their ancient sleep, starting the infestation on the region.

Putting not only Chrysalis's hive, but whole Equestria in danger. Now Queen Chrysalis and her Changeling hive must hold the line and use every resource they had to defeat this a ancient evil, before it consume everything with it's ancient corruption.

The swarm find a way to one of Queen Chrysalis's building camps.

Queen Chrysalis: "The rumors of the Ancient evil were true! What has those fools DONE?!"

Chrysalis was furious and shocked, as the swarm overrun the local guards, causing her worker drones to flee for their lifes.

Drone: "Run, RUN!!!"

Chrysalis stood there. She could flee now and save her remain Changelings. But her pride got in the way. Thinking that the history will remember her as a Changeling, who doomed everyone by her cowardliness. She was already humiliated by ponies and she'll be damned if she let ponies to save Equestria from her own mistake. Plus, she needed a victory to restore her reputation.

Queen Chrysalis: "Cowards! Assemble the troops and push these abominations back to the deeps of Tartarus! I'll not allow any more embarrassments in my name!"

Pushing the swarm's scouting parties away, until the main forces of the Swarm start to arriving, pushing the Changelings to retreat and recoup.

no one is going to write something for you.