I don't think it can be usefully summarized into one punchy word (ETA: I don't think hope is quite the right way to describe what Quirrellmort is missing that's preventing him from creating and ruling over an intergalactic dark empire), but now that I thought for one minute about which one I would choose if I had to pick one, it would be one that doesn't at first brush sound like it fits into that slot at all:

The power the Dark Lord knows not is ambition.