I don't recall getting this before. Most of my characters who actually do the SI main quest have a couple of murders under their belt. I had presumed that my character would not do the SI main quest as per his background, but lo and behold we jumped in. He couldn't take watching Syl torture those poor souls, so he took their punishment. Incidentally, at level 27, she took 18 points off Strength, Endurance, Agility, Luck and Willpower each. I had restoration spells to fix this.
Then she rewarded me with the Manacles of Pain. They don't give me pain when I put them on. It's too bad it's not a zero-weight item. Anyone have a good use for them? Or will it be relegated to a display item in my house?

Nope. I never found any use for them. If you don't mind a murder under your character's belt, Relmyna Verenim is not essential anymore after SI's main quest.