Wow, this is great, one of my favorites of the fest so far. Flitwick is one of my favorite characters, and you really do him justice here. What an inspired pairing, and what an excellent Flitwick backstory. The characterization of Griphook is stellar: uncompromising, believably and understandably unaccommodating, mesmerizing. "I'm not miserable," Griphook scoffs. "I am myself." Yes!!
The sex scene is brilliant. Poor obtuse Neville is an excellent choice of representative for the non-goblin world, and I love the use you make of Gobbledegook, how much it (and the forgetting of it) symbolizes Flitwick's life and choices.
Lines I like:
Filius hesitates — Griphook has spoken Gobbledegook, and it takes a moment to shift his mind into processing it, like cracking open a book that has lain closed on the shelf for many years. An effective image, especially for a Ravenclaw.
Filius kisses him — a soft brush of mouth upon mouth, the way wizards kiss. Griphook flinches, and then bites Filius's lower lip with sharp teeth, as goblins do. Great contrast through parallelism.
Griphook is a wolf while Filius is a dog, soft and domesticated, but he can bare his teeth and show claws too. Great, revealing image
Griphook's nails draw blood, and Filius remembers the exhilaration of pain entwining with pleasure, wonders how he could ever have forgot.
He finds he can't think in Gobbledegook and do this at the same time, it is too much — a foreign language, and yet not foreign — the language of his blood. Perfection. It's all here in this line, all the motifs of self and other and identity and language and memory and forgetting (or choosing to forget).
And that whole section of Filius's dreams of mother and home -- so well done.