[ dryly, seeing how far he can push the truth charm: ] No, I picked a complicated transformation involving many complex anatomical changes that could have dire consequences if botched, when that isn't the magic favored by the Northern spirits, in the middle of a chaotic battleground with many other things going on all because I wanted to test my limits.

—is what I'd say, but since we're all compelled into truthfulness by this charm, yes, I am a dragon 'for real'.

[ it might be a bit more complicated than that but it sure does boil down to it in the end, and isn't that what counts. ]