Tenseiga's Truth

Why did InuTaisho create the Tenseiga? And why did he give it to Sesshoumaru instead of Tetsusaiga? Set between the Black Tetsusaiga arc and his final ascension into the ranks of the daiyoukai, this little piece gives a glimpse of just what might have happened to trigger Sesshoumaru's final understanding of his father's reasons for everything he'd done. Not a pairing story.