Read a couple of stories like that. Don't know if this is the one you are looking for. Anyhow here it goes.
Ultimate Sacrifice by Miwa03
What things will you do for your loved ones? How far will you go to protect them? “A life for a life.” He said. “If you want this Sesshoumaru to bring the half-breed’s life back into the living, then you must do the same. Bring a life to this world.” Kagome was confused "Whose life?" "The heir of the Western Land." He smirked darkly. ~2nd Best Canon in the 2nd Q Dokuga Award 2009 2.0~
www.dokuga.com/fanfiction/story/2082/1
THIS STORY IS RATED MA, NO YIMS