Free magic, as practiced by sorcerers. [There's clear disapproval in Sabriel's tone- not for who she's speaking too, but for those who'd practice such magic.]

Even if they were sleeping, the Dead would still be bound to follow their orders. [Although if the necromancer responsible had died, or been incapacitated... Sabriel hesitates. In that case, what was tying these Dead to Life?]

Or it could be some form of necromancy I'm not familiar with, one that wasn't mentioned in The Book of the Dead.