Prior to 2.4, it had increased chances to Stun on hit.
As of 2.4, it gained a unique affix that, if used correctly, gives player a nearly permanent Vengeance.
Stats (Level 50)
Legendary Hand Crossbow
Calm pervades your soul as you are warmed by the morning’s first light, for you know vengeance will soon be yours.
Energy that shimmers in the morning light leaps from the surface of this crossbow. (pre-2.4 description)