The children know his name, but they don't say it. To say his name is to call him, and you only do that if you really mean it - if you really need him. Those are the rules.
(The adults didn't believe, not at first. Their children had simply been lost, and afraid, and their imaginations ran away with them. Only when enough bloodstains had been found, when enough parents caught their children conversing with shadows, did they begin to wonder.)
He only comes to children - to the lost, the endangered. The ones who can't run to safety are found sound asleep wherever they were being held, small peaceful islands at the epicenter of absolute carnage. They never wake until they're brought somewhere safe.
(The Sleepers, as they came to be nicknamed, were never quite the same afterward. There was always something about them lying just under the surface of their young, somber faces.)
He is not infallible. Sometimes he's too late, and as much as the police have come to dread his summons (crying and screams of children in their ears), they've learned it's far preferable to the crushing weight of dark, formless rage that chases them through nightmares for days.
(One would-be kidnapper actually survived, possibly because he'd had second thoughts and argued to free the boy they'd taken. While in custody he would panic at the sight of any shadow, and had to be kept in full illumination at all times. He was eventually deemed unfit to stand trial and committed suicide a month later.)
No one knows who he is (was.) None but the ones he saves can describe him, and they're young enough that their descriptions if they offer them are difficult to follow. Over time, parents and counselors will piece together a figure from an era their children have never heard of, and there will be enough identical information from enough different sources that no matter how much they want to, they won't be able to question it.
("He's coming," the children would say, and it would be all they had to say to make their attackers stop, and think and realize it was far too late.