Immortal cursed phantom-warriors that can't be killed, only incapacitated for a short time. These things
... F*** yea. Let the world domination begin.
Edit:
First, I'll send one or two each to some huge cities, they'll get incapacitated quickly. Then I'll send a small army to a couple major military outposts. They'll be a bit intimidated, but they'll incapacitate all of the phantoms. Then the ones in the cities will come back, they'll get incapacitated again quickly, but then the news will travel to the military base, leading to the greatest "OH SHIT" moment in all of history as the army comes back to life.
Of course, if they're limited to small areas like they are in Spiral Knights, I might just use them to protect the Amazon rainforest or some shit like that. I'd love to see the look on a poacher's face when they kill an army, think "Hah, stupid natives. Wait, what's with the glowing balls? Where are the bodies?" and then the phantoms come back to life, slice them open, and curse any survivors. Such fun! ^_^