"Isnt this nice then!" whispers in a low tone the now visible skulk (using the hat to take a random skulk appearance) in undercommon.
"A priest will make good sacrifice for the devourer and the Spider will reward me!" he continues, selling the skulk scout story as it might help with the future interrogation of the man.
"Well lights out pretty boy! Tell the Devourer I sent my respects!" he finishes before delivering a few more blows until the man is unconsious.
The without delay and given that there might be others following or the vampire spawns might return he cleans the 'scene' by dropping acid on any bloodstains that might have been caused and a scent-breaker... as much as that might be needed in the sewers.
Then with fast movements he places a pair of masterwork manacles on the hands of the priest and a sack over his head. That done he deposits him in the portable hole and flees the scene after going invisible.