What would happen if a wizard summons a familiar (using find familiar) in the Border Ethereal, and then the wizard returns to the Material Plane (interrupting or ending the magical effects of etherealness)?
Would the familiar disappear due to the distance from the caster, since they are now on different planes?
Does the Ethereal Plane overlap with the Material Plane (Border Ethereal) such that distances are preserved and the familiar remains in the Border Ethereal while the caster is in the Material Plane?
The Ethereal Plane
Its shores, called the Border Ethereal, overlap the Material Plane and the Inner Planes, so that every location on those planes has a corresponding location on the Ethereal Plane. (PHB, p. 301)
Does this make possible the ability of having one’s familiar become an ethereal traveler?