To unlock the endings, you need to clear certain side quest first, below are the list If you unlock all the ten endings, there will be an extra scene after you watch all endings.

Ending	How to Unlock
Ending "A Little Hero"	Clear Sidequest "An Unwanted Reunion".
Ending "A Warrior in Love"	Clear Sidequest "At Journey's End".
Ending "Beast God at Rest"	Clear Sidequest "The Beast God's Birth".
Ending "Iron-Armed General"	Clear Sidequest "Red-Letter Day".
Ending "Life of the Beastkind"	Clear Sidequest "Mankind and Beastkind".
Ending "Overseers of Time"	Clear Sidequest "What Was Inherited".
Ending "Shaman"	Clear Sidequest "Wandering Soul".
Ending "The Lives of Humans"	Clear Sidequest "Assassination of Dias".
Ending "The Show Goes On"	Clear Sidequest "The King's Daughter".
Ending "Valkyrie in Repose"	Clear Sidequest "A Letter to Tomorrow".