This is a little subscriber bonus short trip from Big Finish and finds the 8th Doctor and Lucie discovering an indigenous bat-like species living in some caves.

It's all weirdly reminiscent of ...and the Silurians. There is a 'baddie' bat and a 'reasonable' bat. There is a human being duped into helping them wipe out humans. There are caves.

The similarities are so obvious that it feels like there's a joke I'm missing (aside from what the Erith of the title is).

It also has some odd references to Lucie's first audio adventure, Blood of the Daleks (what I can't work out is why the planet, Red Rocket Rising, or any of the specific circumstances are mentioned - it's all just vague references which seems odd when its a Big Finish story, not a story where they might need to worry about infringing copyright or something).

The Christmassy links are basically the time of year the story is set and nothing more or less - although there is an odd baby being born bit at the end which I think is supposed to tie in with the birth of Jesus.

It's all a bit odd, really.