An Ever-Fixed Mark

You may be looking for the Beauty and the Beast anthology An Ever Fixed Mark.

Fanfiction
Title:	An Ever-Fixed Mark
Author(s):	imperfectcircle
Date(s):	24 October 2009
Length:	14,900 words
Genre:	poly fanfiction
Fandom:	Merlin
External Links:	An Ever-Fixed Mark (AO3) An Ever-Fixed Mark (Livejournal) An Ever-Fixed Mark (Art) cover by chosenfire28
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An Ever-Fixed Mark is a Merlin OT4 story by imperfectcircle (~14,900 words). It was written for the polybigbang 2009. The story cover is by chosenfire28.

Summary: Fusion with Ursula le Guin's planet O. In a world with very different marriage rules, a noble wedding and a magical threat still can't make Merlin, Arthur, Gwen and Morgana talk about their feelings.

Arthur, Merlin, Morgana and Gwen secretly get married because they need to unite their strength to defeat a magical beast.

Marriage Rules

The story includes a note that explains the marriage rules of this universe:

Note for readers unfamiliar with the planet O:

Ki'O society is divided into two halves or moieties, called (for ancient religious reasons) the Morning and the Evening. You belong to your mother's moiety, and you can't have sex with anybody of your moiety.

Marriage on O is a foursome, the sedoretu—a man and a woman from the Morning moiety and a man and a woman from the Evening moiety. You're expected to have sex with both your spouses of the other moiety, and not to have sex with your spouse of your own moiety. So each sedoretu has two expected heterosexual relationships, two expected homosexual relationships, and two forbidden heterosexual relationships.

The expected relationships within each sedoretu are:

The Morning woman and the Evening man (the "Morning marriage")

The Evening woman and the Morning man (the "Evening marriage")

The Morning woman and the Evening woman (the "Day marriage")

The Morning man and the Evening man (the "Night marriage")

The forbidden relationships are between the Morning woman and the Morning man, and between the Evening woman and the Evening man, and they aren't called anything, except sacrilege.

It's just as complicated as it sounds, but aren't most marriages?

Based on these rules the pairings in the story are Merlin/Arthur, Morgana/Gwen, Arthur/Gwen and Merlin/Morgana.