She's the fairy queen, and rules over her own swathe of the night in Britain.

I was reading up on the show to help you with this question, and I have my doubts regarding the other answer, and I agree with Port City about the meaning of 一角. I have confirmed with another native that it pretty solidly refers to a portion/part/corner. I'm posting this answer to give you more to work with.

https://en.wikipedia.org/wiki/Titania_(A_Midsummer_Night's_D...

The character Titania is originally from Shakespeare's A Midsummer Night's dream. Your answer might be in the play for all we know. I wish I had the time to read Shakespeare to help you get to the bottom of this right now, but I can't.

https://ancientmagusbride.fandom.com/wiki/Titania

In this show, she is also referred to as the moon of Tír na nÓg. I found something interesting in the wikipedia entry that might be worth considering.

https://en.wikipedia.org/wiki/Tír_na_nÓg

From the article:

In Irish mythology Tír na nÓg (Irish pronunciation: [ˌtʲiːɾʲ n̪ˠə ˈn̪ˠoːɡ]; "Land of the Young") or Tír na hÓige ("Land of Youth") is one of the names for the Celtic Otherworld, or perhaps for a part of it.

So, there are two takeaways here. She is called the moon of Tir na nog. Tir na nog is not the whole of Britain, but actually the Celtic Otherworld, or, perhaps, part of it.

There is a whole lot of stuff online about Tir na nog, so it would be worth it to look into that some more. You might find some really great ideas to make your translation more authentic while you're at it.

If I think of anything else or get more time to look into this I'll add more information later.

I hope this helps. Good luck with the translation.


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Note added at 3 days 5 hrs (2023-02-05 04:53:08 GMT)
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Yes, I also watched the little introduction Spriggan gave where he called her 夜の主.

It just seems incredibly odd to include 一角 in the sentence when they just could have just left it out and said 夜を統べる.

I'm going to look more into it later, but there are other examples of characters described in the same way. We might be able to find out what is really meant by this phrase if we can compare a few of them. Here is a link to one to start.

https://twitter.com/last_period/status/803841250680045573

I don't know anything about the IP in this twitter post, but in this example, it clearly seems to mean that アンセム家 rules over part of a world or realm called the ブラッドレルム.

I'll see what else I can come up with later.