I really really like this fic, and I thought I should actually comment now that I've read all of it in one sitting, though I cannot guarantee the coherency of this comment as I am rather intoxicated at the moment, sorry. Anyway, I adore the needy violent sex and the state of veiled!Sirius is intriguing, I am looking forward to reading more.
On a random note, The Nature of Instinct was one of the first R/S fics I ever read, way back in the day, so one could say you are responsible for my obsession with this pairing.