The unique affix effect stacks and renews with each application: the potency is added to the remaining one, and then duration refreshes. Any attack suffices regardless of Proc Coefficient and number of enemies hit. Ashnagarr's Blood Bracer does not affect this shield.
Stats (Level 70)
Legendary Hand Crossbow
The Fortress Ballista was famously used by Valla during the Siege of Bastion's Keep. A strangely garbed wizard infused it with a spell on the spur of the moment, apparently conjured from nothing, when it seemed their fate was sealed.