Disclaimer: I own neither the Harry Potter universe nor the His Dark Materials concepts, but I do enjoy putting them together.
A/N 1: This was written in response to the prompt: Harry Potter/His Dark Materials, any, The day their daemon settles is during the first battle at Hogwarts (ala Half-Blood Prince). They don't notice until much, much later.
A/N 2: It's been a long time since I read Half-Blood Prince, so I don't remember the details of the battle, but I like the premise of this prompt.
Coming of Age
Everything happened fast during the battle. There was no time for carefully thought out responses or communicating complicated plans. It was okay, though, because the DA had been training together for long enough that they knew each other instinctively.
If there was no time for the fancy fighting techniques that involved daemons shifting mid-fight or complex charms and hexes, then at least their opponents didn't have time for that either, and their deamons were stuck as adults. The DA's daemons could choose their forms, and they chose them well. They took on the most powerful forms they could, the most vicious forms, the forms they were most deadly with in a fight.
And they were deadly.
Not one of their daemons came away from battle without blood on fangs or claws.
They fought and they killed and eventually they won.
But even winning, didn't seem like winning.
The battle was over, but the war was still going strong. There was no safety, no security. The kids stayed on edge, ready and waiting to be attacked. Their daemons didn't shift, didn't change into the small fuzzy animals they normally would have to provide comfort because there was no comfort in small and fuzzy anymore. The only comfort came in big and strong and powerful enough to defend and attack.
Their daemons didn't want to be defenseless ever again, they didn't want that either, and any adult who suggested they should be, was looked at with too-adult eyes and told "no."
It was choice and necessity that kept them deadly.
It as another year before any of them even considered an alternative and sometime in the interim, the choice had been made final. There was no going back now. They may or may not have settled in that first battle, with blood and smoke and death, but they never again changed after that.