Hobnail's Hammer

What happens when our reality interacts with an alternate reality? When, by accident or design, a door, a pathway, a portal, is opened between the two? Could you use it to cross over? And can other beings use it to trespass into our world? Given that fairies and elves have recently been making incursions into the garden of an 18th century house recently bought by accomplished witch Mary Conway, it would seem so.