From Storm in Heaven:

Touma heard a voice calling to him that day - a voice calling out to him in a dream. That day, Sousuke forsook his family, escaping with the sword that they had guarded for generations. Touma's dreams weren't supposed to mean anything once he woke up; Sousuke's family wasn't supposed to find him or the sword ever again. But once they meet, fate binds them together - and fate has a funny way of sending all such plans awry.