The Faceless Guard are a cadre of spies and assassins, made up of creatures called Shapechangers. These creatures can transform into other creatures they see, and excel at mimicry and infiltration. It is said that this group works for St. Ajax the Invincible.
These creatures are able to transform into and mimic the creatures they encounter. They are resistant to fire damage, and their blood seems to have corrosive properties when in contact with certain surfaces.
“The Changeling” (Episode 6)
A Shapechanger member of the Faceless Guard had infiltrated the Rosso Cielo, and had been waiting for the chance to attack the senior officers of the Chain of Acheron. After putting itself into superposition with both Angel and Judge, it was put down.