"The death of the Banshees."
"They do not name death as 'death'; instead, it is a concept closer to a Clockwork Cat, known as 'slumber.' When a Banshee suffers fatal damage, she transforms into a factor-based elemental body called ectoplasm."
"Ectoplasm cannot be decomposed or dissolved, and at specific times, it can be summoned through special means."
Chapter 830