What cloak of shadows is extremely good for is forcing the opponent to move in close enough for you to reap havoc on the opponent. During playtest games, dropping this card plus an idol of pestilence early forces the enemy to at least close ranks and make a much tighter formation in order to beat you down with. The card is not so good against aggro mages, but even then, if you can manage to keep your distance (maybe a huge wall if bones that you can reconstruct over and over again?). The card is definitely an interesting one, but I think your safest bet would be to run one of each.