Either as a reward at the end of Moon and Star's main quest or alternatively, after the quest, in Ancestor Glade. Once you finish reading the scrolls, head to the exit of the glade to find it. Note that choosing Magebane as a reward for the Moon and Star quest will result in having two copies of the weapon.
A possible reward for completing Moon and Star's main quest.
You will need the Moon and Star Patch provided by the Legacy Patcher for this weapon to appear, the patch is called DBM_MAS_Patch.esp.