Weight: ½ lb.
Estimated Value: 128,000 gp
This figurine is a statuette of an obsidian horse. If you use an action to speak the command word and throw the figurine to a point on the ground within 60 feet of you, the figurine becomes a nightmare. If the space where the creature would appear is occupied by other creatures or objects, or if there isn't enough space for the creature, the figurine doesn't become a creature.
The nightmare is friendly to you and your companions. It understands your languages and obeys your spoken commands. If you issue no commands, the creature defends itself but takes no other actions.
If you have a good alignment, the figurine has a 10 percent chance each time you use it to ignore your orders, including a command to revert to figurine form. If you mount the nightmare while it is ignoring your orders, you and the nightmare are instantly transported to a random location on the plane of Hades, where the nightmare reverts to figurine form.
The nightmare exists for up to 24 hours. At the end of the duration, the creature reverts to its figurine form. It reverts to a figurine early if it drops to 0 hit points or if you use an action to speak the command word again while touching it. When the creature becomes a figurine again, its property can't be used again until 5 days have passed.
Source: DMG p169