Succubus' Key is one of the magical items described in Monster Girl Encyclopedia World Guide III: Sabbath Grimoire.
Guide Book Entry[edit]
A small key made of metal that shines pink. It is used as a "Bedroom Key" by Succubi and other monsters, and places magic on the room that human males are brought into, creating a space that none may enter except for the two of them. A lock is not required for this Succubus' Key, any "room" targeted can be locked, and it will become the sole key that can open it.
By inserting this key made of Mamono Realm Silver into a door, as if the door has become liquid the tip of the key sinks in, and it becomes possible to twist the key to close like when using an ordinary key. The power of the key effects not the door but the entire "room", it protects against all intruders from the outside, for example, even if invading from the door or window, or breaking the walls or ceiling etc. disturbances cannot enter the pink haze. Any room can be used as long as it is a "room", naturally this includes one's own room, all the way up to a room at an inn and the confessionals of churches, all of them become sacred love nests for inma to bring men.
Though it is an extremely strong key, conditions are required to use this key to its full extent. As it is called the key of the "bedroom", the effect only manifests under the condition that two people consisting of a man and a mamono, or otherwise a single man and several mamono enter the room, and it is locked from the inside. This is because the closed space created by the key is preserved by the mana generated by the circulation of mana between men and mamono. Therefore, in the case that there is only one man or mamono in a room the power of the key cannot be used, also, even if a man and mamono are together in a room, if they don't touch at all, or time is spent without joining at all, after a while the magical effect put on the room will become lost. However, as long as the conditions are met and the man and mamono touch each other's body in a closed "bedroom", continuing to have intercourse, then certainly physical keys and destruction mean nothing, and even the archeomancy "Magic of Unlocking" will not apply.
In addition, even if you lose the key in the room or broke it, you can easily leave the room by waiting until the magical effect put on the room disappears. However, for that to happen you of course cannot have sex, and acts like getting intimate together in the same bed are also out of the question, you cannot touch each other so that the circulation of mana does not occur, you have to stay separated. However, a mamono locked together in a confined "bedroom" with just her beloved partner cannot put up with doing nothing. Also, there is a tendency for the mamono and her partner to get the idea "why not just have sex for now?", and because there are many things you completely forget while in the ecstasy of intercourse such as a lost key, in the end, it will take a great deal of time to come outside.