In chains made of gold - Hangmans_Radio (Ao3)

Description:

The creature between them spat at the ground and snarled something in the language of Fallon. Gerard could speak several languages, including that of their enemy neighbours, and he flinched further behind the curtains at his words.

"What did you say?" The guard from the garden could not speak Fallish, but he could understand the tone well enough. He narrowed his eyes at the prisoner, who bravely met his gaze and said the words again, in their tongue this time.

"I said - Fuck your king."

OR

The one where Gerard is the King, and Frank does, in fact, fuck him.

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I fell in love with this one as soon as I read it. It's the perfect mix of angst, pining, lust, and love. There is mpreg in a way because they're mythical creatures, so keep that in mind.