Three Stygian Maids (冥土三姫Meido Sanki, "Three Princesses of Otherworld") is a group of three Tortures maids created by Atlach-Nacha.
Z/X Encyclopedia Entry
Three most prominent ball-joint doll Tortures, combined the finest performances and technologies and released to the world by Atlach-Nacha, who has sold a great number of ball-joint doll Tortures. They can't be obtained simply by money, and their acquisition methods are unknown. Those who obtained them are said to have left this world entirely.
Their beauty and level of killing ability goes without saying. Their devotion are also worthy of special mention. They will invite their master, who has succumbed to eternal life, to death at every possible moment.
All members of this group are named after methods of corporeal or capital punishment.