Heavy Crossbow Dark Spirit is a enemy that appears in Things Betwixt in either NG+ or in NG after burning a bonfire ascetic in the Fire Keepers' Dwelling bonfire. It will be on a bridge-like ledge, shooting bolts at the player with the Heavy Crossbow. At close-range, it will fight with a Shortsword.
It has a chance to drop any 5+ pieces of the Imported Set, though rare. It doesn't respawn after death until you move into the next NG+ or use a bonfire ascetic.