If the Night Rider isn’t mounted, they can use a bonus action to magically teleport onto its Gaunt mount, provided the Night Rider and the Gaunt are on the same plane of existence. When they teleport, the Night Rider appears astride the Gaunt along with any equipment it is wearing or carrying. While mounted and not incapacitated, the Night Rider can’t be surprised, and both they and their mount gain advantage on Dexterity saving throws.