but I can't seem to find it in the forum.

Sesshomaru finds Kagome and she's been split from the rest of the group. He calls her weak and in her mind, she is. He cuts her with a claw and she basically begs him to hurt her and that she's unworthy.

He takes her to his home - the citadel - where he puts her in the capable hands of his second in command (something like that) and heals her. But she is to be trained to be his number one concubine.

There's more, but hopefully, that will ring some bells? I think it was still on FF.net and AFF and definitely MA rated.

Thank you!

UPDATE: Found it!
In His Arms by The Telepathic Hawk.
Definitely, MA rated.

Sesshomaru comes across Kagome alone and broken. What happened? Where's Inuyasha? What is he going to do with a broken Miko who just pledged her life to him?