Manga about a sword that only a little boy can unsheath
You're looking for Hero Tales aka Juushin Enbu. There's also a 26 episode anime adaptation of it.

From ANN:


  Set in a time of chaos, seven heroes of the Stars, the Hokushintenkun, rise up to lead the people. Two of the seven, the Nitenshinson, the Hagun and the Tanrou, are never supposed to exist at the same time, but fate has decided otherwise. Taito, chosen as the Hagun, now must rise up to defeat the Tanrou, as the fate of the Stars destined it to be.