Reaper's Flight: Book 4 of the Chronicles of Aesirium

Book Four of the Chronicles of Aesirium: Two years following the attacks on Oldtown-Against-The-Wall, Aerthos' new Sheharid Is'iin - angelic beings responsible for the transport of souls between the worlds of the living and the dead - has left her childhood home in search of her senior Reaper, a woman she knows only by the name Artifice. But what Rom finds instead is a new type of undead creature - one much harder to kill, and, to her horror, made from her fellow citizens. Against the plottings of the Queen, Rom must once again turn to her friends, knowing that only with their help can they uncover this mystery and defeat the threat these sandmen pose to them all.