Earn the "Through the Looking Glass" achievement during Witch Hunt to unlock this longbow. This achievement is automatically attained in the process of going through the main quest, so nothing special is needed to be done to unlock the longbow.
During Witch Hunt a modified version of the bow can be found in the Elven Ruins. It has 10.00 armor penetration, a range of 44, and is made of Volcanic Aurum. This differs from both the Origins and Awakeningversions that are available after being unlocked.
The rapid aim property reduces aim animation by -1.0s.