An unclean umbral spirit that lends it's senses and intelligence to the unquiet dead. The Black Rider is a demonic spirit who acted as a guide and adviser to a noble or other potentate in life. As part of the pact where the ruler gained the familiar, the ruler swore to be guided "as long as flesh and bone shall endure", quite literally until dissolution of the corpse.
The spirit rides the head, shoulders or highest remaining piece of it's steeds anatomy guiding the undead's movements and attacks. The undead gains exceptional intelligence and will use the spirit's saving throws if better than it's own. The host undead also becomes capable of moving as fast as a human, if it's current speed is less than that and can take full actions every turn.
Inspiration - The Abominations of Yondo