On a different point, the image of the black cowl in box and on website:
The mask looks rather "goblinish" to me, the sort of face on some greenskin minis.
The image in the icon on chest (eye with arrows) I'm not sure of, I wonder if it's an established Chaos or Tzeentch image? I would not play it that way, don't think the Black Cowl would reveal that connection so openly. More it's the symbol of his criminal network "the brotherhood".
Going with the mask being a goblin face has interesting implications in goblin-hating Averland. To help explain the involvement of greenskins (earlier in thread I noted lack of them a shame for Averlander racial bonus mattering), the mask grants a magically compelling power over greenskins and is very intimidating to others (granting or enhancing any Fear/Terror effect).
If anyone sees it and researches it, they discover that the actors at the Sun of Solland (Sulky Sun) have been perturbed by the loss of the "Goblin Mask", one of their props, used in plays about the invasion of Gorbad Ironclaw in particular the tragedy, "Sunset of Solland". It was said that the portrayal of Gorbad by actors wearing the mask was always inspired and terrifying, the theatres had to issue warnings about fainting. The present actors are very perturbed at the mask's loss, it would be good luck for their new production about the Third Battle of Blackfire Pass (a work in progress, yet to be named). There were always some rumours about the mask but those are just theatre superstitions of course.