A very, very enjoyable read! I loved Seamus and Neville here, both. Seamus because his restless frustration is so vivid, and the way he doesn't get why Neville freezes at his kidding at first, and later has no hesitation to let things move to sex. And his pride in and admiration for Neville is so gorgeous. And Neville! OMG, all earnest courage and resistance and leadership qualities, and still bone-deep insecure about approaching Seamus! I adored characterisation in this! And I swear I usually hate piercings (in fic and RL), but here it was as if Neville was proving to himself that he had that little secret rebellious edge, and it worked amaxingly well.
And I loved the way you integrated the story into DH canon. Excellent job, mystery creature!