Please hide WN/LN/manga spoilers.
MAL entry, Anilist. The zeroth episode is a spin-off-ish, focusing on Sylphy after the Calamity meeting with Ariel, becoming “silent Fitz”, and struggling with her new situation; as well as the power struggles between Ariel and her brother I-always-forget-his-name for the Asuran throne.
LN Meta, perversion
I think that they tried to cram a bit too much content into this episode, and as a result a lot of things ended rushed.
Another thing that they cut out is that Ariel is a maso. The bed scene with Sylphie was supposed to awaken her masochism. Still - I love that they set up the dynamic for Sylphy to be the straight woman among two perverts.