Fel-Imbued Artillery Catalyst

Source
This item is rewarded from the following Class Hall missions:
 * Augmenting Our Assault
 * Chink in the Armor
 * Demon Exterminators
 * Go to Fel
 * Resist the Tide
 * Return Fire
 * Coalescing Void
 * Encroaching Gloom
 * Fabric of Reality
 * Memories of the Lost
 * Preserving Memory
 * Their Emancipation

It can also be found inside chests on Argus and rewarded from certain world quests.