Ok, I can't remember who is the slave and who is the master in this fic. But who ever is the master has a myriad of slaves at his disposal but he likes to torture the other part of the pairing more than the others.
Some of the tortures I remember being inflicted are needles, burning, and bdsm type elements. Finn was also a key player in this fic and was forced to watch the tortures.
I believe at the end of the fic the master became really compassionate towards the slave.
I know this is super vague but if you can help out that would be great! Thanks!