At first when they drag him to prison, he feels nothing. There is a numbness, a gaping hole in his very sole where his family should go. Maybe they weren't blood, no. But these people where the family in the life they had built, his brothers. His sister. His godson. All of them have been lost to Sirius, so far gone that he may as well be in another existence entirely. There are times when he finally gives into the bland exhaustion from staring at the dirty rock wall that he passes out into a cold, haunted sleep, one in which he can hear the cries of those he loved so dearly as they are ripped from his heart forever, leaving him at his own fault. He wakes up and at last he cries- the tears are bitter, but at least its something.
The next few days, or weeks, Sirius can't be sure- he moves on to rage. His soul is like a fire, consuming his every waking hour, even as the dementors pass by, he pays them no mind. He feels the hate, long lasting he knows, and pure as any emotion can be. Hate that he is the one who let this happen. That he let someone he loved fool him in such a way- that he was so blind. Hate and anger and rage, they dance within him; and Sirius cries out, screaming and thrashing against the walls for release from his hell. It does not come.
As a month or two has passed, Sirius wonders if his mind is completely gone. He remembers the fights, early ones, stupid ones. Childish things that where James alive and Remus around they would tell him not to think on, as they had not in years- but he regrets it, regrets every second he lost to them for stupid, childish or shelfish reasons. He remembers the bigger fights too- Selling Remus out to Snape for his own revenge on the greaseball, which had nearly cost him both James and Remus, then there had been fights over girlfriends that no longer mattered after Lily... Things that seemed to have made them stronger but now, no now all Sirius can see is the precious time he lost. This is when Sirius wishes the dementors would just kiss him already. He'd rather be without his soul than live like this, without everyone and everything that mattered.
Six months into his lifetime sentence, Sirius stops eating. He loses the will to move, to care that he's not living well- and why should he? What does he have left to care about? To wake up for? Remus has gone on with his life, surely. Harry is with family, he does not need a Godfather, especially not one such as himself. There is no need for him to waste the oxygen he uses, and in his mind, he wishes that breathing would stop too.
The months turn to years, and Sirius barely notices. He eats, when offered, though its not often it keeps him alive. Often he turns into the dog to avoid the terrible guards over the place, and always on the full moon he remains in canine form, howling at the lone orb that he can barely see through the small window in solidarity for his own loss- this is his life now, after all. But even here, in this cell, his own, personal hell on earth that Sirius knows he has earned, he has promises to keep. And he hopes that they're happy, safe, loved and warm. He hopes, because its all he can do.
That is of course, until one day- inspection day- when the higher up Aurors come through with Ministry officials, a trip which they claim is for quality control, but Sirius knows its a corpse count; a tour to see who had finally expired and checked out of their sentence... a witch drops the paper. Sirius waits in the corner, curled up as tightly as he can managed, until they pass. 'Can I even still read?' He wonders, as he picks it up. Sirius finds that it is slow going, but upon unfolding the paper its no longer the words that matter- but a photograph. The Weasley family, he sees, have won a trip to Egypt, or rather the money to go- and with them is a rat- a common sort with long hair in its ears... and a missing toe.
All at once Sirius feels everything again as if it where new, and before he has time to think he transforms and slips through the bars with ease, walking past the dementors with no trouble- after all, what's a dog to them?