The Apprentice's Companion, Book the Fifth
The Particulars of a Single-Entity Plane This type of alternate plane can easily be used as a trap - think, for example, the infamous djinni's lamp or Magira's Chest trick. In this type of plane,  only a single entity may exist at once. If a new entity enters, the old one is quickly and forcefully removed. If no entities are inside, the plane then collapses immediately.