You do need the summoning wand to finish the game.
Did Sammy unobligingly die without dropping it in the west room of the final level (the level where you are told to get the wand from the west room)? If you killed him on the summoning level he isn't supposed to drop it there.
All I can think of other than using a cheat to give yourself the wand (if there is a cheat for that - I never use them so I don't know) is to talk to the guy who sent you to the west room and enter it again and see if anyhing good happens. Assuming <cough> that you have carefully looked (using "alt") already and the wand simply didn't drop.
