Kagome and Sesshoumaru have agreed to mate to help her control her growing powers and defeat Naraku, I believe her powers had something to do with the Shikon No Tama.
After the fight she ends up living with him in the western lands. Even thought the mating was for convenience Kagome is falling in love with Sesshoumaru she is also slowly transforming into a full demon I can’t quite remember if it was because she thought he would love her or if it happened because of the mating.
Well one evening at dinner in the western palace Kagome decides to tell him the good news about her transformation, but before she can reveal the good news he tells her he would like his mating mark removed because he has found a demoness he would like to mate.
She remains in the palace continuing on with the lady of the house duties while the new demoness also lives with them as she is being courted by Sesshoumaru. Kagome starts to lose her mind because she can’t control her demon side without the complete bond.
The ending I remember is Kagome is stuck in some kind of sleep and sesshoumaru won’t leave her side.
If anybody knows this story please let me know I have been going crazy looking for it. I read it about 6 years ago on a single spark.
Thank you