Many an abandoned tower or mansion was once the abode of a Necromancer, whose dark sorceries could only be practised in secret. The Necromancer may long gone, but the poltergeists and revenants remain as any who enter will doubtless discover.
A haunted mansion is normally a building of some kind, but there's no reason why it can't be a ruin, or a cave. At the end of the Shooting phase, all units within 6" of the Haunted Mansion suffers D6 Strength 1 hits to represent the spectral strikes of the mansion's guardians. Due to the ghostly nature of these attacks, armour saves cannot be taken against them. This has no effect on models with the Undead special rule. A unit inside the haunted mansion causes.