I don't remember how it was called in English, I guess it was like "Unbreakable Vow" or whatever, that charm that forced you to do something or die.
Well then, why didn't Voldemort use it with his Death Eaters to guarantee their loyalty?
Had Voldemort used it, his Death Eaters would have spent their days (after his incident with baby Harry) doing their best to find Voldemort, instead of hiding or pretending they were being controlled by the Imperius Curse.
Of course, it is possible that Voldemort did use it with some of them, but they really thought that Voldemort was truly gone forever, so the charm was pointless since Voldemort seemed to have gone. But I still think he didn't use the charm, because, as a minimum, he would have used it with Snape. And Snape undoubtly betrayed him.
Now then, maybe the charm can only be done once? Some kind of restriction? I don't remember any reference about that though...