Spirits of the Breach

Morgana has returned from her year in captivity with Morgause, but things still aren't settled between her and her maidservant.

When she can't sleep one night, Gwen tells her a ghost story, reverting back to old times. But as she weaves the fabric of her tale, a forgotten history is unearthed in the process.

Is Gwen in danger of changing their relationship through her story, or is something darker at work here?
Set in the events of s3 ep1 and s4 ep1, canon compliance for the most part.