Slivers are the result of a failed attempt to kill a member of the Sun. When they have been damaged to the point of physical dismemberment, each piece takes on a demented consciousness. They remember the feeling of being a whole person, and crave it, but can’t remember how to achieve that again. To chase after that wholeness, they devour whole people, burning through their moorings, essentially stealing that victim’s life to feel whole for a while. They don’t eat souls, for the record; they do, however, retain memories, echoes, and full imprints of the victim’s personality and life.
It doesn’t last long. The victim’s echoes burn away, and the slivers go on the hunt again.
A sliver cannot be killed on an individual basis, only contained. The only way to kill a sliver is to bring all the slivers belonging to one member of the Sun together again and allowing them to join, reforming the original being. Of course, then there’s a dangerous, fire-wielding enemy to deal with all over again, so this isn’t always better.
From The Christmas Dragon:
“They are not shadow-eaters,” said Notte (I resisted the urge to point at Pintstripe and shout, Ha!), “although they do, in fact, eat the soul’s moorings. They may not be what you feared, but I fear they are worse. They are every inch as wicked as your child’s mind could imagine, for they are the First War’s leftovers. We call them slivers.”
I smacked myself on the forehead. Slivers! Why didn’t I think of that! But wait, if they were slivers, then that meant gods.
And I was nearly empty. I didn’t have the power to fight gods.
“First War?” said Cassie, eyeing me warily.
“The very first—at least, the first which took place outside of Heaven,” Notte said.
These people were human. They had no chance against slivers. I might have, but not like this. Dear hell, what had I stumbled into?
“What are slivers?” said camo-pants, otherwise known as Sam.
“They are sentient pieces of deities which have been destroyed,” said Notte.