You call forth a Nightmare to serve you. The creature appears in an unoccupied space you choose within range. The Nightmare will serve you as a mount, in combat and out. The Nightmare is friendly to you and your companions for the duration, and follows any commands you issue to it.   If your concentration is broken, the Nightmare is no longer bound by the spell. It might leave immediately, especially if it has been wounded or threatened, but it is just as likely to attack you or an ally, or otherwise gain retribution for being summoned and bound to your service. If you provide the Nightmare with a sacrifice it can devour when it is first summoned, it will be appeased enough to simply return home once the spell ends, without causing harm to you or your companions.