Hey there!
The Hexstaff is one of the treasures from the old ones (From the Albion campaign).
At 55 pts. it gives the bearer d3 extra power dice, rolled each magic phase. If any Hexstaff dice roll a 1 they cause a miscast, and since you might have rolled a 1 one the "ordinary" dices this could possible cause a multible miscast. One for the Hexdice and one for the normal miscast roll.
The hexstaff still has the possibility to draw out dispell dices from your opponent, even if you don't use them. Rather intimidating!
Take care out there!