I haven't come across a 'snape is sirius slave' fanfiction myself. Fanfiction themes can be very diverse, but this one seems rather unorthodox and might not be as common. However, the world of fanfiction is vast, and new works are being created all the time, so it's possible that someone has written it, but it may be difficult to find.
One common theme is powerlessness. Since Snape is in a slave - like situation, he has little control over his own life, which is a major aspect explored in these fanfictions.
One interesting Severus Snape fanfiction where he is in a rather difficult situation like being a slave could be 'The Unlikely Bond'. In this fic, it explores how Snape's character is tested when placed in such a demeaning role. The author delves deep into his psyche and shows how his past experiences and his inner strength play a role in this new and harrowing situation.
In some 'Snape slave to Harry fanfiction', the plot might start with a prophecy or a magical bargain. Maybe a seer predicted that only with Snape as Harry's servant could a great evil be defeated. So, Snape, bound by this fate, has to follow Harry's commands. Along the way, they face many challenges together. Harry has to deal with the power imbalance and Snape has to struggle with his pride. Their relationship evolves from one of forced servitude to a more complex bond of respect and cooperation as they fight against the forces of darkness.
One common theme is redemption. Despite being in a slave - like position, Snape often tries to find a way to atone for his past actions or mistakes. Another theme is power dynamics. It shows how power can be misused and how those in a subservient position, like Snape in these fanfictions, can resist or adapt.
Fanfiction.net is a great place to start looking for Snape slave fanfiction. It has a large collection of stories across various themes, including this one. Archive of Our Own (AO3) is also a good option. There, you can search for tags related to Snape and slavery to find relevant stories.
There are quite a few out there. 'Snape's Forced Subjugation' is one that stands out. It's a long - form fanfiction that really delves deep into the psychological aspects of Snape being in a slave - like position. It shows his inner turmoil, his attempts to break free, and how his past experiences shape his reactions. Another great one is 'The Slave Snape Chronicles', which not only focuses on his situation but also on how the wizarding society views and treats him in this state.
I remember reading 'A New Dynamic' which is a great 'Master Harry Slave Snape' fanfiction. It really plays on the idea of them being in such opposite roles. Harry's character growth as he tries to handle Snape in this new relationship is really well - written, and Snape's begrudging acceptance of his situation makes for an engaging read.
There are countless fanfictions out there, and while I can't point to a specific 'Snape slave to Harry fanfiction', here's how you can find a good one. Firstly, go to big fanfiction communities like Archive of Our Own or Fanfiction.net. Use their search functions to look for fics with 'Snape' and 'Harry' in the title or tags. Read the synopsis carefully. Some might explore a really unique dynamic between the two characters, where Snape is in a subservient position to Harry. But be prepared for a wide range of interpretations and writing qualities. You might have to dig through a few not - so - great ones before you find a gem.
There is also 'Snape's Unforeseen Servitude'. This fanfiction might show how Snape copes with being in a subservient role. It could be interesting as it may contrast his usual proud and mysterious persona. It might also involve his interactions with characters who have power over him, like some of the Death Eaters or even the Order of Phoenix members in a different light.