Azrael’s Soul-harvester form is something Azrael achieves and enters when they they have enough souls/radiation/energy combined. If they are severely angered enough or if have to deal with big threats like Cerberus of etc, they grow in size, horns grow, and they darken like a true reaper. All their ribs extending longer from their chest/stomach and their tail becoming more of a scythe.

Another way of describing the form is Azrael’s true form in some sense but not entirely due to Chimalma’ power keeping them in some stable limit hence them being in their usual “weakened” state more. .her true form is unknown and supposedly vastly more powerful,