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.
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.