Work Text:
Merlin has a child.
She's a tiny little thing with white skin and hair so light it's nearly translucent. She looks at people, blue eyes open wide, and doesn't talk. She hides behind Merlin's legs when people try to talk to her and clutches at the stiff arm at her side like it might disappear.
Arthur gathers from the snippets of information he gets from Merlin that something happened to her when she was young. She is easily frightened and never seems to get a good night's sleep. Neither does Merlin, for that matter.
She's not his daughter. But she clearly is his child in every way that matters. Merlin says she doesn't have parents and that he promised to look after her. Arthur sees the look in his eye and doesn't ask any more than that.
When Merlin's working she'll trail after him like a lost little duckling or stay with Gaius. He swears she'll make for a better physician than Merlin someday.
She's shy with strangers, but she becomes lively when they're alone. She'll run around the room like any other child, except that she isn't as fast, because one of her legs is shorter than the other, and she'll scream and jump and laugh. Then Merlin will lift her up and tickle her and she'll shriek and clutch at Merlin's neck with one arm, burying her head in his shoulder.
Merlin is gentle with her, talking in a low, soothing voice, but he's firm. If she misbehaves or doesn't listen, he crouches down and sternly explains what she did wrong, even if she hisses and growls at him like a cat. He doesn't ever back down until she gives in. Arthur has often thought that he wouldn't be able to do it, until he sees that Merlin has tears in his eyes and is breathing hard as soon as the child turns away. He wants to give her the world, he tells him, let her do everything her heart desires. But then she wouldn't learn anything and grow up spoiled like a self-important prince. And Arthur rolls his eyes at that, but Merlin smiles again.
Soon, she is beloved by the entire castle, just like Merlin is. Gwen always talks to her when they run into each other in the hallways of the castle and starts to spend her evenings in Gaius' chambers, braiding flowers into the child's fine her. Gwaine and Leon treat her like a small royal and Percival and Elyan have taken to carrying her around on their shoulders.
Even Arthur can't help but give in when she starts tugging at his trouser leg and looking up at him with her big blue eyes. It's how Merlin finds them, the child sitting on Arthurs lap, doodling, while he signs state documents.
Aithusa is small and vulnerable, but she's Merlin's. And she grows up loved.