In the story, if I remember correctly, There is a diplomatic or military envoy group sent to the alien world to establish relations or possibly end a war between them and us. One member of the group volunteers to undergo various tortures by the aliens to establish human trustworthiness and in the process gradually loses his humanity altogether.
Piers Anthony, "On the Uses of Torture", from the collection Anthonology. It has the dubious distinction of being one of only two things I have ever read that made me put down the book and think about heading to the toilet.
On the other hand, the torture in the story is not gratuitous, as Anthony explains in the introduction.
The other story that made me feel ill was Stephen R. Donaldson's Gap Cycle.
I have read this one, and though I do not yet remember the title of the story, I can add some details to see if it triggers anyone else's recall (mildly graphic descriptions of torture behind spoiler tag):
The alien society defines status by torment. Their senior leaders are those who have experienced the most torture, inflicted in a specific ritualistic progression, and their folk heroes are those who willingly allowed themselves to be tortured to death.
The protagonist first has an assortment of tortures demonstrated to him, when the alien torturer
subjects the fingers of one of the protagonist's hands to several specific tortures: flaying one, crushing another, prying the joints apart on a third, among some others I don't recall. Later, the torturer inflicts these methods upon other parts of the protagonist's body, including his genitals. The protagonist ends up a physical wreck, far too damaged to ever lead a normal life again.
As the protagonist is taken farther down the path of torment, his status rises until he is the equivalent of a supreme leader to the aliens, with the authority to make the decisions desired by the envoys. The alien offers the protagonist the choice to continue down the path to death, to become immortalized in legend (he says something like 'women will squirt their milk over your grave,'). The protagonist refuses long enough to issue the desired orders. He spends the last bit of the story considering his situation from the perspective given by his experience, which has divorced him from all the values he once held.