Obsidian Steed

See Figurine of Wondrous Power
This polished obsidian horse can become a Nightmare for up to 24 hours. The nightmare fights only to defend itself. Once it has been used, it can't be used again until 5 days have passed.
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.