From Diablo Wiki
The Corpse Raiser is a Reaper that hovers above the ground, but that does not possess extra speed or ability to fly over terrain. Its own ranged attack is very slow and of very low cold damage, but it is unavoidable and always hits. The Corpse Raiser can also create glowing fields of green light over the ground in front from which undead minions emerge (Canine Bones, Hungering Bones, Risen Bones, and Spitting Bones). These are slow, fragile and weak, but may appear in unlimited numbers until the Corpse Raiser is slain. Corpse Raisers do not always raise these minions (this happens in the Contested Villa, for example).
Upon death, Corpse Raiser spawns three to four Vile Bats, identical to normal ones, but spectral green in color. Some Elite Corpse Raisers do not have this ability.
Where to find them