The Rule of Entry

In Barovia, it is widely accepted that undead cannot enter a residence without the permission of someone inside. The origins of this rule are unknown.

Manifestation

Property with closed doors and windows is generally considered to be safe from undead who lack sapience. A Vampire, Dullahan, or other powerful undead entity may request entry.
Aware of this rule, it is common for undead to find ways to bend it. A different window or door being opened, or otherwise accidentally breaking the boundary of residence will allow undead to enter, as they care not for intent. Permission given unwillingly through magical means, such as charming, is also enough to let an undead inside.

Type

Metaphysical, Supernatural