Infernal Tack

From Mordenkainen's Tome of Foes, page 167.

Description
Wondrous item, legendary (requires attunement by a creature of evil alignment).

A narzugon binds a nightmare to its service with Infernal Tack, which consists of a bridle, bit, reins, saddle, stirrups, and spurs. A nightmare equipped with Infernal Tack must serve whoever wears the spurs until the wearer dies or the tack is removed.

You can use an action to call a creature nightmare equipped with Infernal Tack by clashing the spurs together or scraping them through blood. The nightmare appears at the start of your next turn, within 20 feet of you. It acts as your ally and takes its turn on your initiative count. It remains for 1 day, until you or it dies, or until you dismiss it as an action. If the nightmare dies, it reforms in the Nine Hells within 24 hours, after which you can summon it again.

The tack doesn't conjure a nightmare from thin air; one must first be subdued so the tack can be placed on it. No nightmare accepts this forced servitude willingly, but some eventually form strong loyalties to their masters and become true partners in evil.