Darkling's firefly familiar swarm is of the species Photinus carolinus. though not encountered in the mountain that contains his own mine, after he went on the trial of mountain shrines with Txi, Darkling became less resistant towards leaving his own range. on one such tentative excursion across another mountain he encountered fireflies for the first time, and their display fascinated and comforted him. it was an easy choice from there to bring them home with him, to light up the deep dark of his own mine and to make him feel less alone down there.
since fireflies aren't native to the UK, those who venture into Darkling's mine and catch sight of the glows think that this abandoned mine is haunted - which it technically is. Darkling doesn't want anyone to venture into the mine to get a better look, and so will demanifest his fireflies if someone is trespassing around the entrance. but if someone is on their way to getting lost in the shafts and tunnels he'll manifest them in a guiding trail to lead the way back out.
there is a small community of urban explorers and occult fans looking into these helpful 'ghost lights' at the mine, where of course the firefly explanation was explored but dismissed early on. animals on the other hand know what is going on and will sometimes come pay a visit to watch the fireflies with darkling, since they can be found nowhere else in the area.