Curses are the staple ability for any Necromancer, and Blackhand Key gives a good bonus to them, on top of a decent overall skill boost. Its other attributes such as Cast Rate, Life and even the 20% vulpine attribute could come in handy should close quarter combat occur. It also gives charges of Grim Ward, which can also give a bit of a unique style and last significantly longer than a level 4+ Terror effect.